Similitude of 2 images doing a projection between them

Hello everyone, more than a Matlab question, is a mathematical one.

I need to compare two images using a projection and see the similitude between them. The problem is I have no idea how to do it. Do I need to use eigenvectors/values? Or is more complicated/simple than that? I suppose the idea is to convert the 2 images that I want to compare into vectors, and then do some operation, but I dont know really what I need.

I will appreciate any help. Thanks again.