[imported_twinsmom]

Well nevermind. I was able to get help at vbulletin.com.

The trick was to skip the font color box (standard css attributes) on the left (leave it blank) and enter the code in the big box at the right hand side (extra css attributes). The code I entered was: background: #FFFFFF url(path/to/filename.gif);

By the way, you would go to:

Admin CP -> Styles & Templates -> Style Manager -> Main CSS (in the drop down menu for the style you're choosing to work on).

I'm posting the solution I used in the hopes that it might help another newbie at some point.