If you're talking about your sig, just upload the picture, or gif in your case, to imgur
Click on the image, then copy the page url into your sig
After that, put an [ IMG ] tag on the front and a [ /IMG ] tag on the back without the spaces
Example:

If you're talking about an avatar, you'll need to save it to your computer and resize it to 75 pixels or smaller before the forums will accept the upload.
Example:
