I have noticed a strange feature while doing some experiments. Look at the following. My large profile image and small profile image are different!
I can't believe this!
You can see that my profile pictures look differently here and here. Still not convinced? Upload this and check how your profile images are changed.
Why does this happen?
I uploaded this to change my profile image. The result was amazing. Large profile image is almost black, but small profile image is white. It seems that Codeforces' image scaling algorithm is Nearest-neighbor interpolation. Nearest-neighbor interpolation uses data of a few pixels. Saving each image to those pixels separately, we can make two profile images differ