Yes. If you already have an existing .htaccess file, you can just add the code to the file.

Using the following code (as you do) will horizontally center the gallery in its parent container.

<div id="juicebox-container" style="margin: 0 auto;">

However, you may then need to horizontally center the gallery's parent container in its parent container.
Your gallery seems to be nested within 7 further <div> containers and an <article>.
Looking at your web page, it looks like you may need to horizontally center the <div class="entry-content"> within the <article class="post-13 page type-page status-publish hentry"> (using the same method as before).

For your own website's galleries, try creating an .htaccess file containing the following code and upload it to your website's root directory. (Please note that this requires an Apache server and is untested.)

<IfModule mod_headers.c>
Header set Cache-Control "no-transform"
</IfModule>

You could also contact your 3G provider to see if it is possible for content modification to be turned off on your account.

Yes. There is a similar FAQ for SimpleViewer here. Unfortunately, content modification breaks many websites that uses JavaScript, not just Juicebox and SimpleViewer galleries.

It sounds like you are using WP-Juicebox (the Juicebox plugin for WordPress) so hopefully the following information will be relevant.

When creating a gallery with WP-Juicebox, the plugin uses the Media Library image's 'Title' for the Juicebox 'Title' and, by default, WordPress populates the Media Library 'Title' fields with the image filenames.

If you would like to use the post's title for the Juicebox 'Title' in an individual gallery, then you could edit each gallery image in your Media Library and paste the post's title into the 'Title' fields.

Otherwise, if you wish this change to happen globally for all galleries you create (without the need to change the 'Title' fields on individual images), then open the 'wp-juicebox/config.php' file in a plain text editor and change line 69 to the following:

$title_text = $new_dom_doc->createCDATASection(get_the_title($post_id));

Please note that the line number above refers to WP-Juicebox v1.2.0.1.

Thank you for sharing your solution.
That seems to work well across all browsers (and versions of Internet Explorer).
It would be great if valid code was rendered identically in all browsers but this is not always the case, especially in older browsers.

So far, I have been unable to create a stripped-back test case which fails in IE8 by nesting the 'juicebox-container' <div> in multiple parent containers. As long as all the parent containers have heights specified by inline CSS, the gallery is displayed at the correct height in IE8 (and other browsers).

If you were a Juicebox-Pro user, you could use the imagePadding and/or stagePadding configuration options to add padding around a main image when no thumbnails are being displayed. Both configuration options are Juicebox-Pro only (and are not available to Juicebox-Lite users) and take effect when thumbnails are displayed or hidden. The stagePadding surrounds the entire gallery (including thumbnails when they are displayed) and the imagePadding surrounds only the main image.

If using Juicebox-Lite, perhaps the easiest way to create padding around an entire gallery would be to create a border (of the same color as your gallery's background) around your gallery container <div> using CSS.

It would not be possible to create padding around a main image only when thumbnails are hidden by editing a Juicebox-Lite gallery's 'theme.css' file as the logic behind determining whether or not thumbnails are hidden is coded within the 'juicebox.js' JavaScript file which is obfuscated and cannot be modified.

There is a lot going on within your web page and as it loads, you can visibly see elements shifting around until they settle in place (as the JavaScript and CSS is being rendered by the browser).

There are differences in your web page, not just between versions of Internet Explorer but also between different browsers.
For example, in Firefox 19.0.2, your 'Select Language' combobox is initially completely obscured by the gallery (which it is not in IE10, Chrome 25 or Safari 5.1.7) but it partially appears if you resize the browser window and then maximize it.

As you are expressing your galleryWidth and galleryHeight as percentages, check all parent containers of the Juicebox container (your 'box_slideshow' <div>) to ensure that they all have heights specified via CSS and please see the Using Percentage Heights note in the embedding guide. This suggests using inline CSS on the parent containers rather than on the Juicebox container (your 'box_slideshow' <div>) itself.

If there is a problem with the gallery not being able to determine it's height in certain browsers, then you could perhaps express your gallery's height as a fixed pixel value rather than as a percentage.

Another workaround might be to isolate the gallery completely from the rest of your web page by embedding it using an <iframe>, documented as Option #2 here. As your gallery does not use the Expand Button or Back Button, there should be no drawbacks to using an <iframe>. This should avoid any conflicts with your main page and should hopefully work in all browsers.

The link you have provided redirects to http://afrika.delosgaia.nl/index.htm and there seems to be only a SimpleViewer gallery on the page. I see neither a Juicebox-Pro gallery, nor the code you posted (only <div id="box_slideshow"></div>).

I'm not sure it will help (as I have not yet seen the page containing your Juicebox-Pro gallery) but take a look at this FAQ (it relates to IE9 but may also be relevant to IE8):
My gallery looks strange in Internet Explorer 9. Why?

Also, if you are embedding your Juicebox-Pro gallery directly into your 'box_slideshow' <div>, then try removing the 'style' attribute from the <div>, set your gallery's dimensions using the galleryWidth and galleryHeight configuration options (in the embedding code) and, if expressing galleryWidth and galleryHeight as percentages, check all parent containers of your 'box_slideshow' <div> to ensure that they all have heights specified via CSS.

There have been a couple of issues specific to IE8 (regarding the opacity of thumbnail frames and the the FADE image transition type) but these have already been addressed and will be fixed in the next version of Juicebox-Pro.
Just to let you know, there are no known issues regarding gallery sizing in IE8.

If you continue to experience difficulties, please post the link to your Juicebox-Pro gallery so that I can take a look and investigate further.

This is not possible.
The only way to display the grid of thumbnails is to set screenMode="SMALL" but the Large Screen Mode configuration options will work only in Large Screen Mode. There is no way to mix or change the screen modes in a gallery.

A Splash Page is a placeholder for the gallery: an image which, when clicked, displays the gallery itself.
This is different from the thumbnail grid displayed in Small Screen Mode.
If you always want the thumbnail grid to be displayed, set screenMode="SMALL".
If you always want the Splash Page to be displayed, set showSplashPage="ALWAYS".

In order to display a Juicebox gallery, your web page must load the 'juicebox.js' file (which, in turn, loads the 'theme.css' file and other related resources).
However, there is no way to speed this up (other than to perhaps host your images locally rather than have Juicebox fetch them from Picasa's servers). WP-Juicebox embeds a Juicebox gallery using the same method as is described in the Embedding Guide with the exception that your WordPress pages and the Juicebox XML file are created dynamically by PHP when the page and gallery are viewed. If all pages were static, things might run a little quicker (as your server would not need to create the pages on demand) but this is not how WordPress works. The advantage of having the Juicebox XML file created dynamically is that you can change your gallery's images and captions directly in Picasa without having to manually rebuild your Juicebox XML file afterwards.

On this web page: http://www.irmucha.pl/nieswiateczne.html
... it looks like the initial gallery is being displayed by the following code:

$(document).ready(function () {
    doLayout();
    $(window).bind('resize', doLayout);
    new juicebox({
        containerid : 'juicebox-container'
    });
});

... and this instance of Juicebox does not have a valid 'config.xml' file in the correct location.
Try adding the baseURL configuration option (from the embedding code further down your page) to the code above, i.e.:

$(document).ready(function () {
    doLayout();
    $(window).bind('resize', doLayout);
    new juicebox({
        containerid : 'juicebox-container',
        baseUrl : 'nieswiateczne/'
    });
});

I expect that you may not need both instances of embedding code on your page.
Once you have added the baseURL configuration option to the first instance (at the top of the page), try removing the second instance (at the bottom of the page).