depends on how you want it...if you want it to just blend in with the rest of its surroundings then when you make it smaller you just fill it black..i'll post some examples for you
here are some examples that i just made...no..i can do better...
the one on the far left is the one you have.
the one in the middle is simply filled in after it is cropped..(if you want it to blend just right click the image and open paint, and put it on there, and fill it with heavy black).
The one of the right is a quick drawn backround. There are other types, like if you go around and look at other peoples avatars and signitures most of them have backrounds. So you can either take this one or make your own.
Got any more questions?..if you give me more detail I can probably get you a backround myself.
Glad to help!
~Light
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Joined RuneScape in January of 2003. Got first Mime event on July 18th, 2006 - Climb Rope Emote
Second Mime on July 27, 2006 - Lean Emote
I abuse emotes.