If you're running OS X 10.4.11, you won't see a disclosure triangle in a Print dialog; that's a feature which was introduced with 10.5. In Tiger, the Print dialog is already expanded.
What you need to do is to go to the third pop-up menu in Safari's Print dialog (the first two being Printer: and Presets:). By default, this menu is set to Copies & Pages. Set it to Safari (near the bottom of the list). Now you should see a Print backgrounds checkbox.
