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Do you mean in your signature?  You need to have the pic hosted somewhere else since new pic uploads aren't allowed.  Then you need to know the URL to access that pic - the various photo hosting sites all make it easy to get it.  Then you need to modify your signature in your profile using BBCode to link to the picture.

For instance this is the link in my signature which shows my sig pic (note that the curly brackets should be square brackets but I had to change them so this doesn't insert the actual picture here):

{IMG}http://www.rpod-owners.com/uploads/217/rpodandsuburban2.jpg{/IMG}