The member trunk is a storage space for members on Potter's Army.
If you win a graphic award, such as Member of the Month, you can store it in your trunk. Likewise, if your character is promoted to an in character position that has a graphic (like the prefects) you can store that in there too.
Your trunk is also a good place for keeping up with your character's history or testing out new character ideas before you apply for them.
Your trunk is corresponds to your main character name, as stated in the multiple characters list. Before you begin, make sure you are posting in the correct trunk!
Posting Graphics and Awards
I'm sure this is written somewhere else on the site, but if you wish to keep track of your member awards in your trunk, you must host them yourself.
Those of us who make signatures and graphics may erase or move them without notifying you, and then you will get that ugly "this image has been moved" picture instead of your lovely award.
If you don't already have one, the easiest way to host your own images is through photobucket, imageshack or tinypic. Just register an account and you can upload images to your own account via their url.