So you want to create some content for bungie.net? Maybe get a custom avatar out of it? Here's what you need to know:
Themes
To make a theme, you need to download the theme builder. Read through the instructions and then make a cool theme. After doing this, send it to [email]7themes@bungie.com[/email]. More details can be found here.
Avatars
Avatars must be 90x90 and in GIF format. They should be no more than 10KB in size (20KB for animated GIFs). Also send these to [email]7themes@bungie.com[/email].
Inspiration
Check here for user-submitted themes and here for user-submitted avatars. Also, add your own to these threads when you've finished.