Have you tried zapping the PRAM so that the system reverts to its defaults? That should hopefully get rid of that desktop pic you don't want to see. Here are some instructions, in case you don't know what PRAM is about, or how to do it:
PRAM is an acroynym for "Parameter RAM". Pronounced "pee-ram", PRAM is system memory which holds certain software settings, including the current date, as well as information about the current startup disk, and other stuff.
How to identify PRAM-related problems:
The most obvious symptoms which point to PRAM problems, include your computer displaying the wrong time, the wrong date (e.g. 1904), or possibly a blank screen when you turn on your machine. Zapping the PRAM will reset Parameter RAM to its default settings.
How To Zap The PRAM:
1. Press and simultaneously hold down the following four keys during the startup process. You should do this from a cold boot. That is, after you have completely shut down your Mac:
COMMAND + OPTION + P + R
2. Continue holding down all four keys until one of the following conditions occurs:
a. About 20 seconds passes, during which time your Mac may appear to restart.
b. You hear the startup sound a second time.
c. The Apple logo appears and disappears for the second time.
3. Release the COMMAND + OPTION + P + R keys after one of the above three things occurs.
Maintenance Tip:
It is highly advisable to zap the PRAM about every six months or so, in order to keep your Macintosh running smoothly and efficiently.
I hope the above helps to resolve your desktop image problem. Let me know. Thanks! :)