Inappropriate Image Formats for print
Some image formats are intended primarily for onscreen and Web use. Portable Network Graphics (PNG) images can contain RGB and indexed color, as well as transparency. While PNG can be high resolution, it has no support for CMYK color space.
The Windows format BMP (an abbreviation for bitmap) supports color depths from one-bit (black and white, with no shades of gray) to 32-bit (millions of colors), but lacks support for CMYK. BMP is not appropriate for print.
Graphics Interchange Format (GIF) is appropriate only for Web use because of its inherently low resolution and an indexed color palette limited to a maximum of 256 colors. Don't use GIF for print.
JPEG (Joint Photographic Experts Group), named after the committee that created it, has an unsavory reputation in graphic arts. Just whisper "jay-peg," and watch prepress operators cringe. It is a lossy compression scheme, meaning that it discards information to make a smaller digital file.
Assuming an image has adequate resolution, a very slight amount of initial JPEG compression doesn't noticeably impair image quality, but aggressive compression introduces ugly rectangular artifacts, especially in detailed areas (Figure 4.11).
Figure 4.11 Overly aggressive JPEG compression produces unattractive rectangular artifacts. A slight amount of JPEG compression, however, is not noticeable. So don't fear JPEG compression. Just don't overdo it.
Each time you resave an image as a JPEG, you recompress it. Prepress paranoids will shriek that you're ruining your image, and there's some truth to that. While it's true that repeatedly resaving an image with low-quality compression settings would eventually erode detail, the mere fact that an image has been saved as a JPEG does not render it unusable, especially if you use a minimal level of compression. Despite the reputation, JPEGs aren't inherently evil. They can be decent graphic citizens, even capable of containing high-resolution CMYK image data. That said, when you acquire a JPEG image from your digital camera or a stock photo service, it's still advisable to immediately resave the image as a TIFF or PSD file to prevent further compression. However, JPEGs intended for Web use are low-resolution RGB files, inappropriate for print. If your customer provides a low-resolution or aggressively compressed JPEG, there's nothing you can do to improve it.