One of the best things you can do to ensure your printed document looks good is to make sure your image resolution is at least 300 dots per inch (DPI) at the final output size.
Caution: You cannot simply convert a low-resolution photo to a higher resolution by increasing the DPI in your imaging program. The printed result will be a blurry image.
The term resolution is also known as PPI (pixels per inch). It is a measurement of the number of squares (called pixels) of color information available in an inch of space. The more squares, the better the image quality. Below is an illustration of how the same image might appear at different pixel resolutions.