If you mean avatar (the little pic below your user name), the follow Pcrays advice.
If you mean a signature line photo, just copy the URL of the hosting site into the signature line. If you don;t have a hosting site for photos, upload it to the site and open it, copy the url at the top of the page and paste it into the sig line in your profile.
If you have a hosting site there should not be any size restrictions, otherwise you are confined to the upload restrictions of PAFF for the photo page. Match or lessen the photo size and have at it.