Found via Googling "fractal image comparison" - a paper from the Journal of Information Science and Engineering:

... However, when users obtain an image from a newspaper or magazine, they want to retrieve related information about this type of image through a search engine, traditional search engines are incapable of meeting their needs. To help solve this problem, this paper provides an Image Comparison Search Engine (ICSE), and makes use of ˇ§Fractal Image Processingˇ¨ to create a database using image eigenvalues. When users input an image query, this system will generate image eigenvalue data, compare this with the data in the database of image eigenvale, and output the results. Based on our experimental results, ICSE can not only find the exact input image for the source image, but also find the ˇ§right imageˇ¨ when the source image is rotated and misty, and contains impurities...

Disclaimer: this is so not my field, so I've no idea how good / feasible this may be for you.