Create a hash function for images, where each image is a "thumbnail" image of 75x75 pixels

Please don't copy someone else's work. I would like to see someone give their own approach to this problem. Thanks :)

Preferred languages are java, C, C++ or C#

Suppose you would like to build a hash table for images, where the key for each image is a "thumbnail" image of 75 x 75 pixels, with each pixel being one of 256 possible colors. Describe a hash function for a set of such images. Your hash function should be fast to compute and it should strive to map different images to different hash values. In particular, reflections and 90 rotations of the same image should, in general, map to different hash values