Photographs should be scanned at the size required in your artwork at 400dpi and saved as CMYK images (the only exception to the CMYK rule is images for Hexachrome Printing which should be sent as RGB because we can then use the full colour gamut in the conversion process).
Please note: It is important that images are originated at this high quality. Re-sampling an image of a lower resolution (say 72dpi from the internet) to 400dpi does not improve its quality or make it suitable for high quality print.
Monitor calibration is a big consideration when scanning and colour balancing. If your monitor is not calibrated the image you see on screen may differ from the image on the final print.
Where designs are built up using a number of layers and transparencies it is important that these are ‘flattened’ before creating the PDF. Un-flattened transparencies are notorious for causing print problems. Flattening your work will ensure that the finished print is as expected without compromising the creative intent.