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As I understand it, you have a catalog of images and you want to display some of them based on a selection from a
<select> box…is that correct?
Or did you really mean “dropdown menu” which is something completely different?
So, assuming the former, which seems more likely, do you images have various tags/classes/whatever applied that JS/JQ could sort through?
If you already have gallery pages set up, then I’m guessing some sort of AJAX injection is required.