...quark, like indesign, uses a low res proxy image for displaying, in indesign you can turn on overprint preview or change the display performance to high by right clicking on a document. This will also slow the program down with graphic heavy documents...
...for quark 6 in the preferences you have the image options to choose from, in the display section you can specify whether the display of color or grey is to be 8, 16 or 32bit...
...quark 7 is the same, however you also have the option of right clicking an image and changing to the preview resolution to full or low resolution...
...importing tif, eps or jpeg usually display fine when you have quarks preferences set to 32bit display settings...
...importing photoshop eps can be dependant on what preview options are chosen when the image is saved from photoshop. From the the pull down menu in the save as dialog you have different previews to use in the eps. When using macintosh 8 bit or tiff 8 bit preview, the image can display a bit noisy in quark. You can over come this in quark 7 with the right click method and choosing full preview resolution, or saving the photoshop eps file with macintosh jpeg preview...
...which version of quark does your client use?
andrew