Content Based Image Retrieval is a very big topic.
For a brief description about CBIR, you may refer to this article.
Here are also links to some code projects:
1.Finding Similiar Images:
http://www.codeproject.com/KB/graphics/cbir.aspx
2.Automatic Linguistic Indexing of Pictures (ALIP) By Artificial Neural Network Approach:
http://www.codeproject.com/KB/cs/alip.aspx?fid=1532679&df=90&mpp=25&noise=3&sort=Position&view=Quick&select=2850628
Hope these will be helpful to you.
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