How much resolution is enough for images printed on my digital press?

The most appropriate image resolution for digital presses varies somewhat by press, but a good estimate is 300 ppi. As a rule of thumb, you should never use an image resolution close to or higher than the output resolution of the press. Ask your press vendor what it recommends. The exceptions are scanned line art, screen grabs, and other synthetic images as they should have a resolution that matches the final output device (e.g., 1200 dpi).