Maybe they just want to test the feel of it... but you can also do that using your personal picture within your profile without disturbing your post count...:P
Just showing off my sig and Avatar... and a third pic of that guy (picture is featured in my new sig:P):
Yep, it's an edited Baseball Guy from Yoshi's Island. I've recolored and slightly edited his sprites, the blades are self-made though. And he's called Buster, BTW.:D
I pretty much messed the sig up when converting into gif, and I saw the great fail after I closed Gimp (shame, could have saved a lot of work if I saved to xcf, too...). Anyway, it still looks good IMO.