First of all not that I can't fined your http://soundmaster.ucoz.org/728.gif (like jackass mentioned above), but I can't even access your website at all, I get message "SITE NOT FOUND". Are you sure you gave us correct address of your website, if you did, then are you sure that you registered that website. I would suggest to go to your uNet.com profile and see if soundmaster.ucoz.org is registered with that uNet acc. If it's not then you need to register it first.
Just in case if everything works out for you or if the other users asks the same question, here is the step by step how to change the background of that table.
To change your background go to:
Control Panel » Customize design » Global blocks » Top part of the website
Look for this line (This code located on the 1st line)
<table cellpadding="0" cellspacing="0" border="0" width="100%" height="100%"><tr><td width="5%" style="border-right:1px solid #333333;"> </td><td width="90%" valign="top" style="background:url('/.s/t/531/1.gif') #9498A1; border-right:1px solid #FFFFFF;border-left:1px solid #FFFFFF;">
You want to replace Blue url with your own background image.