Images for offset/digital printing should be of 300 dpi, but you can get away with as low as 220 dpi if that's all you've got. Any more dpi is a waste and just makes the files size larger. Under 220 dpi and you will start to see the pixels in the print.
Remember recommended dpi refers to the effective
resolution, not the resolution number given in Photoshop. If you have a
300 dpi, 5x7 image that you use in a layout at 200% the effective
resolution is only 150 dpi, not enough for good print quality. On the
flip side, if you have a large 72dpi image (meant for web) that you
scale down to 25% of its original size; the effective resolution is 4
times the file's dpi This means your document resolution should be set
to 300 dpi to allow for the extra resolution needed for the images. Most
layout programs include the dpi settings in the preferences for the
program (edit>preferences) or, the resolution is set when you make a
new document (file>new document).
Terms to understand
- Pixels per inch (ppi) - Pixels are the individual elements that make up a digital file.
- Dots per inch (dpi) - A measure of output resolution rendered by printers