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.