Image Hash Improvements

[Webreaper]

It might be convenient to be able to force-generate image hashes (used to detect dupes) without having to regenerate thumbnails from scratch.

Also, the thumb generation code uses the large thumbs as a basis to create smaller thumbs if they exist. This means we'll generate the hash from the large thumb, rather than the original file, which will cause the hashes to not match with images where the hash was created from the original image data.