DCS, EPS, JPEG, or TIFFS are OK.
Never JPEG a DCS file.
Never place a JPEG file in Illustrator, as it won't always work. There is a work-around by replacing it with an EPS file.
Crop down images to approximately the cropping size in the document.
Total ink density should not be lower than 300% and up to 350% UCR.
Total ink density in any one area must not exceed 340%
All images must be CMYK unless they are Duotones or Tri-tones
All Duotones and Tri-tones must have the exact same colour names as the native document file.
Do not have extra channels in files.
Create all close-cuts in Photoshop as Clipping Paths.