first off, your current avatar isn't working.  this is what you had as an address:  http://www.kmprod.com/traditional_mini_golf.jpghttp://www.kdregio.hu/kerek/images/minigolf.jpg

notice the mistake.    i cut out the address i assume you meant.    http://www.kdregio.hu/kerek/images/minigolf.jpg

so you almost had it.    and for the cartoon avatar site...no, you don't need to be a member of their forum.  just right-click over the avatar that interests you and choose properties.  copy the http:// address and paste into the avatar URL box.  also, type in the width and height also given under properties labeled: dimensions.  that's pretty much it. 

and for animated .gif's just do a search.  that's what i did.  or if you're talented like ks, spud, or bud, you can make your own.    hope this helps

lastly, yes, you have to set the pixel width and height.  the maximum for both is 100.  i set yours to 100/96