How do I let my members upload .gif images to use as their avatars onto my server?
Right now they can upload their own avatars to my sever, but every time they upload one with animation, the animation does not show..
You've currently only got PNG or JPG as options for uploadable avatars.
You can tell them to host it at any of the free image hosting services (google) and allow them to link external avatars. I'm not sure if you have 'Download avatar at given URL' set for external avatar it will upload them as their current format.
How do I get it so that my members can upload .gif animated avatars?
If the animated gif was too large and you chose to resize it, it will only take a frame.
Same if you choose to have them as a png.
You can't resize an animated gif by GD2.
To large as in height and width or to large as in the size of image (like bytes, etc., etc.)?
Either way, you cannot resize an animated gif using that process without loosing the animation.
You're only option is to manually upload it for them, or the method I mentioned earlier.