On Multiverse normal users are limited to having 100x200 avatars, however if you donate and become Brony + you will be given the ability to have much larger ones!

To have a rotating avatar set up, simply sign up for an account here: Signavatar

Then, add whatever images you want to your rotation (they will have to be within your avatar size limits) and give a staff member the link. The link should look like this: