heh, i have been trying to do the same thing, forcing users to print the background images when they print. there is really no good way about it unfortunately. more difficult solutions involved activeX controllers, which only work for IE 5.5 +. Only option is to,

In Fire Fox, in print preview, check the page options and 'turn on' print background.
In IE, under Tools>internet options>advanced 'turn on' print background.