I think you can make them any size.
Here's /lego/, for example: http://infinitycup.shoutwiki.com/wiki//lego//Roster
>I have no idea if that's where to upload
There's an Upload page on the left sidebar to upload an image to the wiki, then you edit the Roster page to ads those uploaded files in (see the /lego/ roster page source for easy reference). Let me know if you need a hand.