Another thing to watch out for is that the avatar must end in '.jpg', '.gif' or '.png'. I don't think '.bmp' is allowed.
46 x 46 is the maximum size supported by the new MyCE styles, especially if it's animated. Anything larger will either fail or be rescaled down
Note that the older 64 x 64 size is no longer allowed, even with the older V1/V2 styles. Anyone with this size will retain it, unless they later change their avatar. However, even those who had larger avatars before the new restriction came in will still have their avatar rescaled to 46 x 46 when viewed on either MyCE style.