Currently GIF images in the gallery shows up as a red x when viewing as thumbnail, if you use GIF format for your signature you can still use it, just click the red x thumbnail and the full version will show fine, then just rightclick the image and use the url in your signature with the [ IMG ] tag.
It will work fine.
Anyone looking at the gallery and see a red x just click it and the pic will show.
This will be fixed in a later patch for the gallery software, it’s a problem with the GD library in PHP.
For now just do as mentioned above or stick with JPG format.