There is no universal solution, there are a few algorithms like SURF [1], SIFT [2], Winsurf [3], dhash (the one presented here), ahash [4], phash [5] each concentrating on a particular feature of the image and has it's strong points and weak points.

[1] http://en.wikipedia.org/wiki/Scale-invariant_feature_transfo...

[2] http://en.wikipedia.org/wiki/SURF

[3] http://www-db.deis.unibo.it/research/papers/IWOSS99.ABP.pdf

[4] http://www.hackerfactor.com/blog/index.php?/archives/529-Kin...

[5] http://www.hackerfactor.com/blog/index.php?/archives/529-Kin...