Strict hashsums (like MD5 / SHA-1 / SHA-2) don't make too much sense here, as the images from the 'less obvious sources' might be thumbnails of different sizes.

Strict hashsums (like MD5 / SHA-1 / SHA-2) don't make too much sense here, as the images from the 'less obvious sources' might be thumbnails of different sizes. Or, probably, the same size but reencoded (not always a good idea, but happens in the practice). Would a fuzzy hash, like the output from a known and fixed classifier work for that?