Mark, If you're talking about an avatar (like my phonograph), or a photo in your signature (like my Browns helmet), John is correct that you need a web host. Go to
Photobucket, sign up, and upload your photo(s) to your album. Once uploaded to Photobucket, copy the URL of the photo to your clipboard.
Then,you need to click on "My Home" in the horizontal menu near the top of each forum page, scroll down and click on "Personal information, email, password, etc.," then scroll down to either "Signature," and/or "Your photo." Follow the instructions for posting an image, at the link John posted.
To wit:
Type [image] then paste the URL to your photo directly after that, then type [/image]. Leave no spaces between any character or letter.