JPEG Format The Joint Photographic Experts Group

(JPEG, typically pronounced “jaypeg”) format is a compressed format typically used for photographic images.

JPEG Format Specifications:
Unlike GIF format images, JPEG images store full RGB information, so they can contain up to millions of colors. Ordinarily, that would increase the file size enough to create prohibitively slow downloads. However, the JPEG format compresses images more efficiently than GIF format, often resulting in files one tenth their original size.

However, the compression method used is lossy, meaning that some of the original data from the file is discarded when it is saved. The format is designed to discard the data that is least likely to be noticeable, and you can choose several levels of compression. The more the file size is compressed, the more information is lost. If you save with the maximum quality setting, the JPEG image is virtually indistinguishable from the original, but it is much larger in size than files saved with a lower quality setting.

You can save files as progressive JPEG images; this option displays a series of increasingly detailed images when downloading. This allows the viewer to get a general idea of what the file looks like faster than if it was displayed at high resolution first. This feature is not supported by all web browsers, but the two most popular web browsers (Netscape Communicator and Microsoft Internet Explorer) support it. Lastly, JPEG images can hold ICC profiles, so if the web browser software supports color management, the image can be adjusted automatically to convert its colorspace to the viewer’s.

Color space information can be embedded within JPEG files. If the web browsing software that displays the file supports color management, it can compensate for the system gamma and other color space parameters of the computer that is being used, so the image should appear correct regardless of the computer that is being used to display it.

Best Uses for JPEG Images
As mentioned previously, JPEG is the best format for retaining the color fidelity of photographic images. However, it is not as efficient at saving grayscale images as GIF format. Since grayscale images only contain up to 256 gray levels, the GIF format is better than JPEG. The JPEG format is not as good as GIF for illustrations. The JPEG compression algorithm can create “artifacts,” or anomalies, in flat color areas and small text. Since the GIF format is lossless, flat color areas and small text remain as they were originally designed.

One easy mistake to make is to repeatedly save the same file in JPEG format. Since the format loses some information each time you save the file, each successive saved version will be lower quality than the previous, and the artifacts can become very noticeable. To avoid this problem, you should always keep a copy of the original RGB image for future editing, rather than editing a JPEG file directly.

Most of the above information is from a course I participated some years ago but the company does not exist any more

In a newsgroup discussion February 26 Year 2002 I quote:
But the real question is not TIFF vs. JPG. After all, TIFF images can include the same compression as JPG. The real question is JPG/JFIF compressed images vs. lossless formats such as the commonly used lossless TIFF. You *can* display lossless compressed images on the web in PNG format, and they will be pixel for pixel identical to lossless 24-bit Tiffs.

I always use PNG for lossless images on web pages discussing JPG compression. See, for example, one of my own pages
(Warning: the images on this page total 1.5 Megabytes. There is a reason for this. Detailed images stored in lossless format have large file sizes. Please be patient. )

You might also find some useful information hear about file format for archiving your digital images