How do I use avatars on MySpace / Facebook?
MySpace
After you uploaded one or more images (see the official MySpace FAQ on how to do this) just go to your change photos page. Hit a "Set to Default" button next to an image to make it your avatar.
Go to "Edit Profile" and select the "Picture" tab. Then use the Browser function to select the images you want to upload and finally hit "Upload picture".
Note: Remember that both MySpace and Facebook got limitations regarding format, file size and content of your uploaded images.
Please read their official statements on this matter.