Is there an algorithm to compare several pictures? I'm looking for an algorithm to store small dimensions of some pictures and later search on the stored data for matches similar to a new picture. Maybe something with Self Organizing Map in machine learning ?
It looks like you are looking for algorithms for Content-Based Image Retrieval. As long as you don't expect any miracles, you should be quite happy with a colour histogram approach. Have a look at MPEG-7 colour descriptors.