There are two types of image file types that the web recognizes: JPEG or JPG) and GIF. (There's also the PNG format but that's another story)

JPEG is a format that compresses an image's resolution so that color takes precedant over sharp details. GIF compresses the color (actually it simply reduces the available colors down to 256) so as to retain more details in lines. Think of JPEG as being the best for high color images such as photographs and GIF as better for clipart or diagrams with less color

When you're ready to add graphics to your Web pages, you'll immediately be confronted with a question: How should you format your images? Graphics in their natural state are too big to be quickly downloaded over the Web, so you have to compress them. The standard compression formats are GIF and JPEG (compression, by the way, doesn't shrink the dimensions of your graphics, it just reduces the size of the file).

But is GIF or JPEG the better format to use? The resounding answer to this question is: It depends. It depends on the type of image you're working with, how small you want your image file to be, and the way you want your graphic to download.

One of your primary considerations is the type of image you're working with. Photographs and graphics with lots of color fields, and particularly colors that blend and fade into one another, are best served by JPEG. For example, an unnerving picture of your co-worker waiting to sing "Yankee Doodle" in a Club Med talent competition should be formatted in JPEG (see below). If, on the other hand, your image has flat color fields, like the talent show poster, it will compress well in the GIF format.

The reason for choosing JPEG for images with more complex color patterns is that this format enables you to save images with millions of colors, whereas the GIF option restricts you to 256 colors.

Another important issue is file size. JPEG permits a greater degree of compression than the GIF alternative, enabling quicker downloading times for larger graphics. And JPEGs appear to retain almost complete image quality for most photographs. As a demonstration, this photo showing the producer of Webmonkey at a staff meeting has been compressed as a JPEG (above) and as a GIF (below). Those dots you see in the GIF are called "dithering," and they come from that format's characteristic adjusting of pixels within a graphic to simulate the display of colors not in the GIF's color palette.

GIF creates a table of up to 256 colors from a pool of 16 million. If the image has fewer than 256 colors, GIF can render the image exactly. When the image contains many colors, software that creates the GIF uses any of several algorithms to approximate the colors in the image with the limited palette of 256 colors available. Better algorithms search the image to find an optimum set of 256 colors. Sometimes GIF uses the nearest color to represent each pixel, and sometimes it uses "error diffusion" to adjust the color of nearby pixels to correct for the error in each pixel.

GIF achieves compression in two ways. First, it reduces the number of colors of color-rich images, thereby reducing the number of bits needed per pixel, as just described. Second, it replaces commonly occurring patterns (especially large areas of uniform color) with a short abbreviation: instead of storing "white, white, white, white, white," it stores "5 white."

Thus, GIF is "lossless" only for images with 256 colors or less. For a rich, true color image, GIF may "lose" 99.998% of the colors.

JPG is optimized for photographs and similar continuous tone images that contain many, many colors. It can achieve astounding compression ratios even while maintaining very high image quality. GIF compression is unkind to such images. JPG works by analyzing images and discarding kinds of information that the eye is least likely to notice. It stores information as 24 bit color. Important: the degree of compression of JPG is adjustable. At moderate compression levels of photographic images, it is very difficult for the eye to discern any difference from the original, even at extreme magnification. Compression factors of more than 20 are often quite acceptable. Better graphics programs, such as Paint Shop Pro and Photoshop, allow you to view the image quality and file size as a function of compression level, so that you can conveniently choose the balance between quality and file size.

RAW is an image output option available on some digital cameras. Though lossless, it is a factor of three of four smaller than TIFF files of the same image. The disadvantage is that there is a different RAW format for each manufacturer, and so you must use the manufacturer's software to view the images. It is reasonable to use this to store images in the camera, but be sure to convert to TIFF, PNG, or JPG immediately after transferring the image to your PC.

BMP is an uncompressed proprietary format invented by Microsoft. There is really no reason to ever use this format.

PSD, PSP, etc. , are proprietary formats used by graphics programs. Photoshop's files have the PSD extension, while Paint Shop Pro files use PSP. These are the preferred working formats as you edit images in the software, because only the proprietary formats retain all the editing power of the programs. These packages use layers, for example, to build complex images, and layer information may be lost in the nonproprietary formats such as TIFF and JPG. However, be sure to save your end result as a standard TIFF or JPG, or you may not be able to view it in a few years when your software has changed.

TIFF is, in principle, a very flexible format that can be lossless or lossy. The details of the image storage algorithm are included as part of the file. In practice, TIFF is used almost exclusively as a lossless image storage format that uses no compression at all. Most graphics programs that use TIFF do not compression. Consequently, file sizes are quite big. (Sometimes a lossless compression algorithm called LZW is used, but it is not universally supported.)

PNG is also a lossless storage format. However, in contrast with common TIFF usage, it looks for patterns in the image that it can use to compress file size. The compression is exactly reversible, so the image is recovered exactly.

Currently, GIF and JPG are the formats used for nearly all web images. PNG is supported by most of the latest generation browsers. TIFF is not widely supported by web browsers, and should be avoided for web use. PNG does everything GIF does, and better, so expect to see PNG replace GIF in the future. PNG will not replace JPG, since JPG is capable of much greater compression of photographic images, even when set for quite minimal loss of quality.

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