UPDATE - compare images on the fly
OK, I got very intrigued by all these algorithms, so much so, I spent many many sleepless hours these last few days trying to get my head around them (already 1am here!)
I've got something I think is pretty darn smart in comparing images here...
Please give it a whirl with your own images and let me know what you think....
Try to make them as different as possible....
http://borkedcoder.com/image_comparison/
Report back here your ideas + suggestions....