It is possible to edit your profile. On the page where this is done, you van edit your name, forum title,Forum signature and above all: BIO. That is the place where many many people place the codes for the banners.
Problem is however that the square in wich you can type is so terrible small. I think Groundspeak will please thousands of cachers when they programm this square a lot bigger, say a halve page! It will make editing a lot easier! Since I'am certainly not the first to ask this, the question is: why isn't this done already?