Almost without exception artwork downloaded from the Web is 72 dpi (extremely low resolution). This art is designed to look great on a computer monitor, but prints ragged and fuzzy. Typical file formats for Web art are JPEG (JPG) and GIF. Don’t use them for print.
Photographs made with digital cameras are growing in popularity. The image files they produce use JPEG compression so that the file size is small. This form of compression destroys the image for print. lf you must use a digital camera choose an option that puts the least number of photos on one roll of film so that the data loss won’t be quite as great.
High end prep houses produce excellent digital photography. Before you contract for the work, make sure the images you buy can be reproduced in print.
Some stock art photography purchased on CD is in JPEG format. If you have a choice between Hi Res or Low Res versions choose the Hi Res version. When the physical size of the image is very large and will be reduced for printing, the results are usually pretty good. Also remember that these images will likely be in the RGB color mode, and will need to be converted to CMYK if printing in full color, or Grayscale if printing in one color.
File formats that are typically of poor quality for print are PICT, WMF, DCS and BMP. Print a 600 dpi laser proof. If it looks bad, the printed version will also.
Using printed pictures as originals to scan presents a problem. They already have a halftone screen applied to them. When you scan them you apply another screen to the image, resulting in a strange looking pattern called a Moire. If you must scan a pre-printed image, and if your scan software allows it, apply a blur (or Soften) before you scan. Otherwise apply just enough of a blur in your image editing program to fade the Moire pattern.