Debut theme images opening in a new page

[lisaRIOP]

I'm using the Debut Theme and, like I've noticed a few people have had, am having trouble with my product images. When I click on the thumbnails, the window switches over to a page showing only that image, and I need to click back to return to the listing. I want the thumbnail to just show enlarged in the same image area as the main picture.  Shopify have said it's to do with my theme code and suggested trying out the new Dawn theme instead, but it's giving me the same issues.  Anyone got any ideas?

thanks!