Hey there.
The posters you are referring to are called signatures. You can find the rules for signatures
here.You can request to have a signature made for you in a forum called Graphics Central. You will not be able to view that until you are a first year. (To become a first year you should keep posting.)
Once you get your signature, save it to your computer and then upload it using sites like photobucket or tinypic. When it is uploaded it will give your a list of links. The link you need to use is the direct link. For example, mine is:
CODE
http://i890.photobucket.com/albums/ac107/brighid_2010/RR/curatorsigcopy.png
You can add the signature to your posts by going to My Control Panel > Edit Signature. When you enter the signature it should be wrapped in image code. So it should look like this:
CODE
[img]http://i890.photobucket.com/albums/ac107/brighid_2010/RR/curatorsigcopy.png[/img]
Then click on update my signature.
If you want to get a sig before you reach first year you can make a request from
The Dark Arts in
Forum Requests.I hope this helps. If you need anymore clarification please feel free to PM me.