How to measure similarities between two images

[qqilihq]

See here for some ideas:

Find original source image for s thumbnail

Hi there, I have a bunch of thumbnails and want to find the original source image which has a much higher resolution. Did anyone do this successfully and can guide with some hints how this might work? I came across the feature calculator for images but I am not sure about how this might work for images when they have different sizes… Many thanks and best regards Andy