First: There's a little button labeled "Edit" that you can click on in the upper right hand corner of one of your posts if you want to go back and add more information to it, or correct it.
Second: The Imperial Ranking thing is a sign of how many posts you've made. I don't know how many posts equals what rank, and please don't make a thread about it. If you're really curious, just PM one of the moderators.
As for changing your avatar: move your cursor over to the "Home" link at the top of the page, but do not click. When the drop-down menu appears click on "Your Account." From there click on the button labeled "Your Info." Scroll down to the very bottom, and you now have two choices for your avatar: Number one is to select an image from the gallery. To do this, simply click the "show gallery" button.
If you want to link to an off-site avatar, you must first host it on a free hosting site.
http://www.photobucket.com
http://www.imageshack.com
Both of those sites work fine for hosting avatar images. Once you've uploaded your avatar image onto one of your picture accounts, highlight the URL for that picture and copy it. Then paste the URL into the box above the "Save Avatar" button and you're golden.
One last note: If your avatar is too big, people tend to whine. My massive "TOFU IS THE MASTER OF THE UNIVERSE" sign got me yelled at, primarily because it was too big to be displayed on most monitors. (It's a joke. Don't worry, eventually you'll think I'm hilarious. That or you'll want to kill me, like most folks 'round these parts opt for.)
If you want to change the size of an image on photobucket, just click on the "edit" button and select the "avatar" measurements for your image. As for imageshack... I have no bloody clue how to work imageshack, primarily because I don't have an account there.
Welcome to the forums, and enjoy your time here.
