Avatars are resized for use in the forum threads and converted to jpg which don't support transparency.
The resizing also introduces artefacts.
Graphics such as the one you have used don't lend themselves to being resized too well due to scaling/interpolation.
If you click on it to view your membercard or go to your profile, you will see a better quality version.
Avatars can be up to 192x384 pixels so try experimenting with different sizes.
Well, it may be worth to convert avatars to JPG, but it would nice and more convenient to have an option in the AdminCP to switch the conversion On/Off. I see in many forum users are crazy about their avatars and some of them like moving avatars.
I'm not sure how easy/difficult that might be, but I think you should take it into account.