I tried importing an album image that did not have 1:1 aspect ratio. When I imported it with 'Aspect Ratio Fixed' checked or unchecked, the image showed up in iTunes as if the aspect ratio was not fixed.
But it appeared correct in the Mp3Tag preview ("Extended Tags..."). However, even though it appears to have 1:1 aspect ratio in the Mp3Tag tag editor ("Extended Tags..."), the actual dimensions are shown in text and it was not 1:1.
Is this a bug with the software, or the tag source script?