Your gallery seems to function OK on its own (www.spoken-image.com/clients/7559LL/Gallery1/) so I suspect the problems are related to the other code on your web page.
But now the 'expand' doesn't show a large image (is that a simple link gone wrong?)
When expanding your gallery and navigating between images, the main images seems to be pushed to the very right of the gallery container. This is likely to be caused by global CSS rules on your web page which the gallery has no option but to inherit and which may be interfering with the gallery's own CSS rules.
Your web page loads 12 external CSS files. Try temporarily removing them from your web page and re-introduce them, one-by-one (checking your gallery after each one), until you find the source of the conflict.
the link to the gallery doesn't work (again link gone wrong?)
I think this may be due to one of the HTML errors on your page (an unclosed <script> tag).
Change:
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"</script>
... to:
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"></script>
Juicebox searches through the <script> tags on the web page to determine the 'jbcore' directory (in order to know the location of other files) and this broken script tag may be interfering with this process.
With regard to the Splash Page being large, all that can be done to reduce this in side is to reduce the dimensions of your gallery or scale your web page for mobile devices using the <meta> 'viewport' tag .