So far what I've found out:
Files cannot be of different types than specified!!!
Sprites are 64x64p Name.gif
Card fronts are 459x687p Name card.jpg
Card backs are 459x687p Name card back.jpg
Card portraits are 64x64p Name icon.png
Item icons are 64x64 Item Name icon.png
Banners are 480 x 90 Bannername banner.png
Screen grabs of event info are arbitrary sizes but have to be Eventname announcement.jpg
http://fantasicawiki.com/edit.php is for creating a new unit or monster. Always check to make sure unit/monster with the same name has not been created (Gaap, Gaap 8, Gaap 9 as examples) or that the unit/monster in question does not already exist.
Creating a page find the best fit previous event and copy paste edit to fit.
I have not yet found a way to delete files that I uploaded of the wrong type, and I cannot change the file extension. Picking up on the nuances of the wiki syntaxes is quick: see "Powered by MediaWiki" button on bottom right of page to find out more information.
To change the name of something mislabeled, without changing the file type, the only way I've found to do this is to use the Move tool.