To optimise your images you’ll need to consider the final size your images will be used at. Photographs and images should be 300dpi at the size you are going to use them.
72dpi images are used for the web but will not print well. It’s important when you’re purchasing or using an image to think of the end result and if it’s going to be used for both web and print buy a 300dpi image and resize it for the web and not the other way around!
There’s no point taking a postage stamp sized image at 300dpi and then blowing it up to a A4 size. Conversely, photographs at more than 350dpi will have no effect on the actual printed quality and will unnecessarily increase file size and processing time.
Image settings for best results
All images should be converted to CMYK, and then saved as psd TIFF or JPEG files
Photos should be scanned at 300dpi at 100% size
Line-art images should be scanned at 800 to 1200dpi at 100% size
IFF and EPS files are saved without any image compression (not JPEG, LZW or ASCII encoded)