Image downsampling

I'm sort of curious -- there are a lot of filters and masks out there, but I have heard very little about automated or algorithmic image detection, insofar as being able to, for example, determine that a portion of an image has been downsampled -- or to detect if an image has been obviously corrupted, without needing to visually preview the image. This sort of capacity would be quite useful, but I can't seem to find any references to that or any closely related topics when I research on Google. Anyone with some image analysis experience have a word or two on this subject?

You should create a feature vector of an image.

Considering how many authors downsample an image before releasing it, I cannot possibly fathom how this will help.