If you have the gif then you should upload it to sites like photobucket. Don't use tiny pic because it will not work. When it is uploaded you will see some of the codes copy the one that has [IMG] at the beginning of it then click where it says "profile" here on this site then select "forum profile information" at the left side of the options that appear and go down to where it says "signature" and paste the code for your gif in there and hopefully it should work.
