Upper right. Hold your mouse over your username next to the envelope and the flag icons and you'll see Your Profile Page with links to change all those things.
Upper right. Hold your mouse over your username next to the envelope and the flag icons and you'll see Your Profile Page and click on Signature. Click on the image icon located to the immediate right of the smiley icon and paste the URL of the gif that you have hosted on an image hosting site such as photobucket/imageshack/etc.