how to find similarity between two imges or two audio signals using matlab?

I want to find similarities between two images(to give a secure place for watermark) or two speech signals(for key word spotting)?

[Walter Roberson]

Let me see... the similarity between this platypus and this chameleon is... right there, the slightly darker spot on each marks their cloaca !

Answer 1

[Youssef Khmou]

there are many statistical tools that can be used for this purpose , ( rmse, Correlation, cross spectral density, ...etc), try the pairwise distance for example :

 >>doc pdist