On a related note - XnView has a handy feature whereby you can tell it to search for duplicates or similar images in a folder structure and/or selected folder structures.
The simplest setting is 'same file name', next one up is 'same file data' (which is what I guess most dupe finders do), the third is 'same image data' (I've never used this, but I imagine that it's to say "these images are visually identical"), the fourth is the similarity search.