Could it be that it's a GIF?, Maybe it's not compressing it and the other browsers aren't recognizing the encoding. I'd try a JPEG to determine if this is the problem.
From Imagemagick docs
Note that since Unisys claims a patent on the LZW algorithm (expiring in 2003) used by GIF, ImageMagick binary distributions do not include support for the LZW algorithm so GIF files are written uncompressed. Users are recommended to use the PNG or JPEG formats instead, and should consult the Unisys LZW web page before enabling LZW support.