He means your avatar image.
Basically, you take a screen shot of your character in the game whenever you want with the Prt Scrn key. Then, paste it in Paint and crop the image by moving the sliders around the borders on the image to show only what you want to be seen. After that, save it and upload it to a image hosting site and set it as your avatar image.
As for his sig, Yuki does an epic job with everything he does, so... Yeah.
Oh, and random discussion thread would be cool.