oh I see the confusion here, everybody is thinking that it is a company computer (as daffy654 pointed out in the first sentence of their first reply). This confusion has been caused by the use of the phrase "the Systems Administrator" which would normally mean somebody else in a company that job is to look after the computers. In this instance as it is your own computer you are the systems administrator.
When you log into a computer you can either login as a user or an administrator, the users are normally locked out from preforming some tasks (such as changing desktop backgrounds) whereas a being logged in as a administrator allows access to everything and you are obviously logging in as a user, you should have another password for you computer to enable you to login to it as an administrator (it may be the same password as your user one).
What you need to do is login with the username administrator and the correct password for this account (as said may be the same) and change the settings for your user to enable the background to be changed. If you could post back with what version of windows you are running somebody will probably give further details on how thats done.