Click on usercp
Click on Edit Profile
Go to the signature box, and enter the following in:
[ img ] http:\\\\the website where the picture resides [ /img ]
And that puts the picture in. But there shouldn't be spaces inside the brackets, but I had to type it that way so you could see it. Hope that helps.
One thing you have to be sure of is that the website where your picture is allows remote viewing. The one most recommended is www.imagestation.com, which is free to use. I know that Geocities and Tripod do not allow remote viewing on their sites.