Are you talking about the picture that appears next to your comments and in your My Stuff area? If so, look for the "change picture" button below your current picture in your My Stuff area. When selecting a picture from your computer, make sure it's pretty small. To use a picture from the internet, first save it to your computer. Right-clicking on most internet pictures will give you a menu that lets you save to your computer.
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If you intend to show it on the forums, you have to upload it to an approved host (the list is on a thread in the announcement forums), then paste the link between [img] and [/img].
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