Yodar - Welcome to the forum!
In the upper right hand corner of the screen, you will see a box with your name and something that says "Private Messages". Click on that. On the left side of the screen you will see things like edit avatar and edit signature. You can up load directly, but it's best to use a picture host like imageshack or something to resize for standard Avatar settings, then cut and paste the link for your avatar and signature files.
Hope that helps!