Gary,
First you have to upload the picture from your computer. The file size must be 100Kb or less. If you're unsure how to do this, do a search...there's quite a few helpful posts out there.
As soon as you get done uploading the picture, click on it to view it. Copy the URL from the address box at the top of your browser...you'll need it in a few minutes. Next, you'll want to get into your profile to update it. In the "Avatar Image" section of your profile, on the right hand side, click on "Select different avatar image." When the window opens, it's got a Custom URL field that you'll need to paste the URL that you copied from your picture. After you paste it, click the "Set URL" button and you should be good to go! Next step....POST AWAY

Hope this helps.
-jokester