What statistical features can I use to distinguish these images?

I have two 25x25 pixel images that are quite similar:

A: http://imageshack.us/photo/my-images/546/18lwin.png/ B: http://imageshack.us/photo/my-images/254/18hwin.png/

except, image B has a white vertical line running parallel to the edge of the two objects, and image A does not. Are there any statistical tools that I could apply to both of these images to automatically tell me that this property exists?

I have tried histograms, FFT, and plain row-wise plotting of the pixel data, but I can't see what features to extract from this data in order to distinguish the images...

Thanks for your advice!