differences between images

i have to make a program that finds differences betwenn images in raw files. the raw file contains byte type data and i need to find where are differences and make a circule round it. i know how to make the circule. i also know how to find one different byte but i have to find sets of different bytes next to each other, cause that is a difference. any one knows a good algorith? or something that may help me?

http://www.google.co.uk/search?q=image+processing+FAQ
Image processing is some serious maths, you'd better be ready for some long reading sessions.

Where are the images coming from?
If it's a camera for example, then any small change in position or lighting will pretty much change every single byte in the file by some small amount.

it's just for images in the gray scale. tks