Be sure your supplied images are high resolution: 300 PPI for grayscale or CMYK images; 1200 PPI or more for line art (also known as 1-bit). Anything less will produce poor, fuzzy or pixilated results. Graphics and photos taken from the Internet (PNGs, GIFs or JPEGs) are discouraged, due to their low resolution and potential copyright issues. Scaling down an image improves print quality but scaling up an image will only reduce the print quality and therefore is not a solution to low resolution images.
It is best to supply images in CMYK color mode. RGB images will be converted to CMYK, but be aware that color shifts may occur.