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You've set "Avatar image max width (pixel)" and "Avatar image max height (pixel)" to 100 in Admin >> System Admin >> Preferences >> User Info Settings ?
If so, are you running XOOPS 2.0.5.2?
I just tested this on XOOPS 2.0.5.2 (my setting is 80x80), and it worked correctly. It allowed an 80x80 image, but rejected images with dimensions 81x81, 81x80 and 80x81.