you might have to have a certain amount of posts before you can use this feature.
if not, here is how you do it. i don't know how to do it from a url, so i'll tell you the only way i know. find the picture you want and save it to "my pictures".
go to user CP up in the left hand corner right under the forum "masthead" or "logo". click "user control panel", under "settings and options" click "edit signature". there will come a window you can write in - scroll down and you will see "signature picture". click on option 2 "Upload Image From Your Computer" click "browse" til you find it. then click "save signature".
hope this helps, but for some things you do need a certain amount of posts. (i think 50, but it may have changed to 100).
good luck.
p.s. i don't think you are allowed to put a picture of a living person.
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