Image Resolution for Your Print Job

It is important to note that a low-resolution image cannot be made into a high-resolution image. There is no way to add pixels to improve the quality of an image. When in doubt, always work with high-quality images.
The most important element to consider when printing images, is how large they can be while still maintaining quality and detail. PPI (pixels per inch) refers to the pixels contained in a digital image. DPI (dots per inch) refers to the number/density of ink dots printed by a press.
In general, images must have at least 300 dpi for high-quality print resolution.