Hi... Once logged in, the path would be to click "Profile" on the top navigation bar. On the left navigation menu, click "Forum Profile Information". Select "I will upload my own picture". Click "Browse" and look for the correctly sized thumbnail image to upload. Then click "Change Profile" on the bottom.
It is important to knock down the image size to the correct (small) file size so the forum will upload the picture. Large pictures are restricted. You can also host your images on a personal web server. Note that the forum administrator may choose to block uploading at some point, and these instructions may not work. If that is the case, then profile images will have to be hosted on a third party web server.