You can take pictures using the Xbox Live Vision Camera and use them as your gamer picture. Apart from this, I don't think there is a way to add your own gamerpic. I would imagine this was done to avoid the issue of offensive gamerpics, etc.
Alternatively, if you don't want to buy any from the marketplace, you can unlock a large variety of them playing games. There are also free gamerpics available for download every now and then - you just have to have a look about and see what you can find.
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