The picture you want to upload to your sig must first comply with the sig size which is no more than 600 pixels wide and no more than 150 pixels tall. Once you edit your pic to those specs, you need to find a hosting site to upload your picture to. www.photobucket.com
is a great place to start and its free!! EDIT: LINK FIXED
Create an account, upload your picture then click on the img link and it will self copy. then on the forum when your logged in, click on the profile tab up top and where it says sig, paste the [img] [/img] link there then submit.
your sig should appear on the bottom of every post.