Hi,

you could use something like this (the image for the background):

background: url(../App_Themes/CorporateSite/Images/leftmenuBg.gif) repeat-x left bottom;

There are more examples in the CSS stylesheets of default sites. You could find them in Site Manager -> Development -> CSS Stylesheets.

I hope it will be helpful for you.

Best Regards
Helena Grulichova