1) the "leader" of AO would be Head Administrator CyberGig1, if you hit the View Forum Leaders link at the near bottom of the Forums page you'll see a list of all the staff and you can PM any of us any time.
2) Junior Member is just your current status as time passes and you get more posts your title will change. Upon reaching 500 posts you may change this title to whatever you want it to be. If you try to spam your way up to 500 I garuntee you'll be banned long before you reach it.
3) Yes you can add your OWN artwork, pictures, and wallpapers to the gallery, however uploading things that aren't yours will result in them being removed and you being in trouble.
I'm also moving this to Forums and feedback, every thread you've made today was in the anime/manga section THIS IS NOT THE ALMIGHTY GOD SECTION OF THE FORUM. Look at the other forums and figure out where things go, this is why we have more than one section.
Finally, PLEASE go read the Rules and forums F.A.Q. all of these and more can easily be answered there.
http://www.animeonline.net/showthread.php?t=2312