It depends on what avatar you're talking about. If it's something someone made, like a graphic or icon, then you either need their permission to use it, and/or you need to give them the credit for having made it. If it's just a regular picture of Ayu, like the regular scans/photos posted here, then you can just use it without having to edit it. Another main thing to remember is to never hotlink to pictures in your signature.
