This is a known issue QuarkXPress 7 and 8 when running under Mac OS X Leopard.
It can be addressed by installing a system level EPS preview generator.Download and install the EPS Preview Engine for Mac OS X Leopard. This system utility is available at the following URL: http://www.eternalstorms.at/utilities/epsqlplg/index.html
Another way to resolve this issue is to install the Quick Look Utility for QuarkXPress, available from QuarkLabs. This is available at : http://labs.quark.com/
Once you have downloaded this utility, place it into the following location and restart your computer: Hard Drive:Library:Quick Look