It looks like your gallery's Splash Page image is being squashed by the following custom CSS code from line 154 of your 'style.css' page:

.content-box .item img{display: block;width: 100% !important;height: auto;-webkit-transition: all .5s ease; /* Safari and Chrome */-moz-transition: all .5s ease; /* Firefox */-ms-transition: all .5s ease; /* IE 9 */-o-transition: all .5s ease; /* Opera */transition: all .5s ease;}

... specifically the width: 100% !important; entry.
This custom CSS rule is being applied to all <img> tags within .content-box .item (which is where your gallery is) including those in your gallery.
Normally, Juicebox center-crops the Splash Page to fill the viewport but your custom CSS overrides this and forces the image to be displayed with 100% width no matter what size the Splash Page is.

First of all, try removing !important from width: 100% !important; to see if this helps. (This might be all that is required to solve your problem.)

You could also target your custom CSS to only those elements on your web page that require them using further CSS selectors.

Another possible solution might be to use the following CSS:

.jb-splash-holder img {
    width: auto !important;
}

Add it to your web page after your 'style.css' file is loaded (maybe at the end of your gallery's 'jbcore/classic/theme.css' file) so that it is used in preference to your own custom CSS for the Splash Page image.