If the background-image property you have set in a stylesheet displays on PC, but not on Mac, make sure you haven't accidentally specified the height or width of the div without a unit designation (i.e. no px or %). I do that sometimes (accidentally harkening back to html properties) and it will break the style on a Mac, in Safari, Netscape AND IE 5.2. Works OK on a PC, though, on IE 5.5 and 6.

In other words be sure it reads:
width:1024px;
height:768px;
not
width:1024;
height:768;

If that doesn't solve your background-image display problem, make sure you specify the style as

background-image:url(http://whatever.com/image.gif)

with NO single or double quotation marks around the path.

Did you remember to upload the image to the server?

If all else fails make sure you have your semi-colon at the end of the line!