if you want it in your sig then you get your gif and upload it to a host site like imgur then got to your sig settings and click the upload picture icon, then you put the url of the gif in the little bar and thats pretty much it, remember to keep the gif no taller than 250 pixels
if you want it in your avi then you have to buy the extension in the shop, if you did buy it then just go to your avi settings and upload your gif, make sure its 100x100 pixels or it wont work (unless you get the extension for it then its 150x150 i think)
Unfortunately it's not possible to use it in your profile page, but if you want it in your signature you find the gif you want, either save and upload to Imgur or get the direct link, then post as such with the image code you want inside of the IMG tab.
Just go to settings and the signature bar.
Plop the image in your signature (example: [IMG]http://tenhourguy.com/tom_250px.gif[/IMG])
Please remember that your signature must be no more than 250 pixels tall plus a line of normal-sized text. You may wish to centre your image or put it into a table to put text beside it if it isn't 700 pixels wide.