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Just go here and click browse and find the image location on your pc and click it then click upload. After it loads it will give you options of many ways you can use the URL. Copy the one that says "for forums and message boards" and paste it in the text box of your signature in user CP.
If you are talking about the pics below some of the member's posts, that area is call a "signature". Mine will show 4 vettes, license plates, and a few other items.
To get to that area, go to UserCP, Edit Signature.
Read this thread to learn how to post pics on the forum.
There is a testing section of the forum also where you can practice posting pics