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As Steve indicated, you upload an 'avatar' from your system to the Nikoncafe server through the form at the bottom of your profile. There are some restrictions on size (height & width as well as file size), but they are covered on your profile page.
As far as the avatar goes, I found it helpful to "Save for Web" in PS CS. (Actually, more than helpful. Before I did that, I could not do it!) Look at the size in the lower left corner as you adjust the quality.