Paint reads your avatar as a 256x256 image.
Paint.NET reads your avatar as a 256x256 image.
GIMP reads your avatar as invalid.
IE10 reads your avatar as invalid.
Firefox reads your avatar as a 16x16 image (??????).
Chrome reads your avatar as a 256x256 image.
Windows' "Details" inspector reads your avatar as a 64x64 image.
It sounds like the DIB header in the BMP states that it's a 64x64 image, but the BMP pixel array has 16x too many values.