.GIF (Graphic Interchange Format) files use a maximum of 256 colors, and are best for displaying noncontinuous-tone images or those with large areas of flat colors, such as navigation bars, buttons, icons, logos, or other images with uniform colors and tones.

.JPEG (Joint Photographic Experts Group) file format is the superior format for photographic or continuous-tone images, because JPEG files can contain millions of colors. As the quality of a JPEG file increases, so does the file size and the file download time. You can often strike a good balance between the quality of the image and the file size by compressing a JPEG file.

.PNG (Portable Network Group) file format is a patent-free replacement for GIFs that includes support for indexed-color, grayscale, and true-color images, and alpha channel support for transparency. PNG is the native file format of Macromedia Fireworks MX. PNG files retain all the original layer, vector, color, and effects information (such as drop shadows), and all elements are fully editable at all times. Files must have the .png file extension to be recognized as PNG files by Macromedia Dreamweaver MX.

.TIF, .TIFF Tagged Image File Format. Used to exchange documets between different applications and platforms. ....

.pbm Portable Bitmap file format. A lowest common denominator monochrome file format originally designed to make it reasonable to mail bitmaps between different types of machines. Now it serves as the common language of a large family of bitmap conversion filters.
.pcd Kodak Photo CD file format. Used to store scanned photos. Each .pcd file contains a number of scans of the same image at different resolutions.
 .pct Picture format. Popular Macintosh graphics format used with page layout and graphics applications for transferring documents between applications. See .pic, .pict.
.pcx ZSoft IBM PC Paintbrush file format. One of the oldest bitmapped formats popularized by MS-DOS paint programs that first appeared in the early 1980's.
.pdf Portable Document File format. Adobe document reader that allows portability. Often used on the Web for better control of page layout.
.pds Planetary Data System format. Created by the Planetary Branch of NASA to store solar, lunar, and planetary data collected both on Earth and by spacecraft.
.pic, .pict Picture file format. This format is popular with many image programs, including Lotus PIC Graphics, Micrografx Draw!, Pegasus Imaging Corporation, and Video Show.
.psd Native bitmap file format of the Adobe Photoshop graphical editing application.
.ps, .psid PostScript file format.
.psp Paint Shop Pro image file.
.pub Native file format of Microsoft Publisher, a program that aids in creating publishing materials.
.pict Picture file format. Popular Macintosh graphics format used with page layout and graphics applications for transferring documents between applications.
.pif Program Information File format. Stores additional information about executables that are foreign to the running multitasking system such as ressource usage, keyboard and mouse virtualization and hotkeys.
.pit Macintosh PackIt format. Archives that were pioneers in Macintosh file compression and archiving but today are obsolete.
.pnt MacPaint format. Similar to PC paint applications.