OK, that's the most obvious one down then!
If you have established that it's just Photoshop pics try changing the picture mode and the file formats to see if you can print greyscales or duotones, make sure there there is no compression or 'clipping' on the files.
If it's any kind of picture that refuses to print, it's more likely to be a Quark problem, again in v.4 + look at the picture box info, check the 'clipping' setting and then the page setup/print dialogue box there is an omit tiff/eps option which needs to be unchecked also