When looking into filtering, I have been wondering if I can make a filter on a value such as product.images.size (the number of images attached to a product)? Is it only price?
Most themes support the ability to add custom filters with metafields. We just released our theme - Paper with this feature.
I believe the closest you can get to filtering based on number product images would be to use another app/tool to automatically add metafields based on image count - from there you should be able to add a filter based on these metafields.