Image Compression - Image compression is the application of data compression on digital images. In effect, the objective is to reduce redundancy of the image data in order to be able to store or transmit data in an efficient form. en.wikipedia.org/wiki/Image_compression

Run-length encoding - A simple data compression scheme in which sequences of the same item are replaced by one such item and a count (so for example the text BBBBB is stored as a single B with count of 5) en.wiktionary.org/wiki/run-length_encoding

Lossless Compression - Any method of data compression that guarantees the original data can be reconstructed exactly, bit-for-bit.

Lossy Compression - Compression in which image data is lost and the image cannot be retrieved in its original form.

JPEG (Joint Photographic Experts Group) JPEG is a standardised image compression mechanism. JPEG is designed for compressing either full-colour (24 bit) or grey-scale digital images of "natural" (real-world) scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or black-and-white line drawings (files come out very large). JPEG handles only still images, but there is a related standard called MPEG for motion pictures. JPEG is "lossy", meaning that the image you get out of decompression isn't quite identical to what you originally put in. The algorithm achieves much of its compression by exploiting known limitation of the human eye, notably the fact that small colour details aren't perceived as well as small details of light-and-dark. Thus, JPEG is intended for compressing images that will be looked at by humans.

GIF (Graphics Interchange Format) The Graphics Interchange Format was developed in 1987 at the request of Compuserve, who needed a platform independent image format that was suitable for transfer across slow connections. It is a compressed (lossless) format (it uses the LZW compression) and compresses at a ratio of between 3:1 and 5:1 It is an 8 bit format which means the maximum number of colours supported by the format is 256. When should I use JPEG, and when should I stick with GIF? JPEG is not going to displace GIF entirely. For some types of images, GIF is superior in image quality, file size, or both. One of the first things to learn about JPEG is which kinds of images to apply it to. Generally speaking, JPEG is superior to GIF for storing full-color or grey-scale images of "realistic" scenes; that means scanned photographs and similar material. Any continuous variation in color, such as occurs in highlighted or shaded areas, will be represented more faithfully and in less space by JPEG than by GIF.