Require some minimum resolution for images so we won't have all that pictures too small
Added on Aug 15 14:16
Its quick and easy to upscale images. Given the number of upscales on Google Images used to get page hits, I doubt this requirement would do much good. And sometimes there are no high-res versions of images.
Not everyone has great eyesight that enjoys these types of sites.
So when the pictures are too small, they lose their impact.