There are two image types used for web pages,  GIF files, and JPEG files.  You will need to know about both types of images to take advantage of all the web can do for  your students. 

Compression

Why did the first generation of web browsers use new image formats instead of exisiting formats?  One reason is that GIF and JPEG files are compressed, meaning that their file size is smaller than might be the case with the same image stored in other formats.  We all appreciate compressed images every time we view a web page that contains images.

GIF stands for Graphics Interchange Format and was originally developed for users of CompuServe.  The names of files stored in GIF format usually end in .gif.  GIF files use what is called LZW compression. 

How GIF Files are Compressed

GIF files are compressed by replacing long strings of data with shorter ones that indicate the length of the long string.  For example, if your image has a blue background, there is no need to store the information separately  for each blue pixel.  Instead, a GIF file stores the information to make the blue color and the number of times to play the blue pixel, so the image can be much smaller.   In LZW compression, none of the information in the picture is lost when the image is produced.

GIF files can only store information about 256 colors.  This is adequate for clipart, banners, diagrams and pictures you can draw, but is not so good for artwork or images where detail may be important.  Most images produced with low-priced digital cameras will display adequately in GIF format.

Jpeg                                                                          Gif

JPEG stands for Joint Picture Experts Group.  The names of image files stored in JPEG format end in .jpeg or .jpg.  Like GIF files, JPEG files are also compressed so that they are small enough to be downloaded rapidly over the web.  Images captured by digital cameras and pictures are traditionally stored as JPEG files.

How JPEG Files Are Compressed

JPEG files are usually compressed into smaller files than a corresponding GIF file, so they load faster than the corresponding GIF file.  However, to make JPEG files smaller, some of the picture information is lost so JPEG compression is called lossy.  JPEG files can store more than 16 million different colors, many more than the 256 colors that can be stored in GIF images.  JPEG images can be made to fade in (with some image editors) but have no option for transparent backgrounds.

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