Is it possible to set padding for the picture when thumbnails are hidden? On lower resolution screens the picture touches the bottom of the screen and I don't want to reduce it's resolution any more.
I guess it could be done in the theme.css but don't know where.

Yes, this is it. I didn't realize that the common embedding code should not be used at all when using the "Resizable Gallery with a Header" source code. It already has embedding code.

Small suggestion: if just the support for "Using a Resizable Gallery with a Header" was a bit more specific about what to do then maybe I wouldn't make this mistake. It shows the example but I couldn't find any information how to use it. Found somewhere in the forum that I should get the source code.

Anyway many thanks for great galleries and great support.

This is only because your page uses the XHTML 1.0 Transitional Doctype Declaration.
If your page uses the HTML5 Doctype Declaration (like the sample 'index.html' page in the 'web' folder), then you would not get this error message.

Changed to HTML5

Your page loads the 'juicebox.js' file twice (from two different locations):

<script type="text/javascript" src="./nieswiateczne/jbcore/juicebox.js"></script>
<script src="swiateczne/jbcore/juicebox.js"></script>

Try removing one of them to see if this makes a difference.

Done. I could only remove the second one from the body part. If the one from the head section is removed then auto-scaling function to fit the picture between header and footer stops working.

Also, it should not make a difference but try renaming your XML files to the default 'config.xml' and then remove the configUrl option from your embedding code.

Done that too but as you said, it did not make a difference.

So unfortunately I still sometimes get the error.

I'll join this topic hoping because it's still the same problem.

And I'll start the same way: "I tried everything..!" (I think). Followed this topic and found mistakes in my code, fixed them. Checked the code in http://validator.w3.org/ as well and the only errors that it shows are in the code that's generated by JuiceboxBuilder-Lite - it expects "type" for <script>.
Anyway, I have two galleries under two separate folders so I used baseUrl. Then I tried renaming the gallery files from config.xml files and use configUrl.

Still the same result : Juicebox Error: Config XML file not found happens sometimes (even though it's not Config.XML any more). BUT it does load it if I refresh. Sometimes one refresh is enough, sometimes needs more.

First gallery: http://www.irmucha.pl/nieswiateczne.html , second one: http://www.irmucha.pl/swiateczne.html