By looking at your source code, wordpress is pulling your customized CSS code. It seems that you did modify your current stylesheet by copying the entire CSS code, modified the #mainpic selector and pasted the 'new' code back in the Stylesheet editor.
Get rid of whatever code you have in the CSS Editor, and paste the following code:
background: #fff url('http://img145.imageshack.us/img145/631/mainpic01mu3.jpg') no-repeat;
Make sure that's the only code you have in there. Now, check the "Add to existing CSS (View stylesheet)" radio button. Save.
This should fix your problem.