The screen name should be the character name you want to create--you can have multiple MV characters if you want to participate in multiple empires or just group your games by a theme.

The serial number is what you used to register your game, if you did. If you use Stardock Central, you can get your SN emailed to you by using the Lookup Serial Number command on the Tools menu.

The picture is optional. Some folks (like me) use the same pic for our forum avatars and our MV character. Some folks don't bother with a picture. Others have one pic for their forum posts and another for the MV character(s).