I got a problem similar today, it appears 3.1 cannot use ImageMagick when converting gif to gif, you need to keep the default of converting gifs to pngs.
Rules[]=image/gif;image/png;convert
instead of my preferred
Rules[]=image/gif;image/gif;convert
This is a shame as it worked no problem in 3.0, well its only a problem if you are using large images on a mac, which has a real problem with large png images..
Tony
Tony Wood : twitter.com/tonywood
Vision with Technology
Experts in eZ Publish consulting & development