If this is the viewer I'm thinking of, this isn't a problem on the demo page or for a typical installation. So there's possibly something about your thumbnail that's different that's causing it.
Short answer:
Lower the z-index style of the thumbnail or raise the z-index of the larger image.
Long answer (if you want more help):
Please post a link to a page on your site that contains the problematic code so we can check it out.
In the future please link back to the demo page of the code you're using so we don't have to guess.
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