There are three main different kinds of image file which are suitable for use on the web: GIF, JPEG and PNG. When you're putting images on your website, which file type you decide to use will depend on the purpose for which you intend to use the image.

GIFs

GIF is an 8-bit, 256 colour file format that supports animation. The name can be pronounced with a soft or a hard G, and stands for Graphics Interchange Format. GIFs are extremely popular on the web because they are supported by virtually all web browsers. The compression used in GIF images is lossless, meaning that sharp edges are retained. This lossless compression algorithm was patented in 1985, and this led to a dispute between the patent proprietor, Unisys, and CompuServe, in 1994. This controversy resulted in the development of the PNG standard as an alternative to using GIFs.

Because of the limited palette of GIF images, "dithering" can be used (where two colours are "blended" using a chequerboard pattern) to make gradients seem smoother. However, this will increase the size of the GIF file, which will defeat the object of using a GIF if it is being used for its high compression. It also results in the image having a grainy look. Optionally, one colour in the 256 colour palette can be designated as transparent.

GIFs are suitable for simple images which do not use many colours, such as logos or text. They are also useful for small, simple, repeated animations, particularly animated diagrams (rather than for movies or videos). They are not suitable for colour photographs or any images with continuous colour because of its limited palette.

JPEGs

JPEG (usually pronounced "jay-peg", and standing for Joint Photographic Experts Group) is a form of compression whereby the ratio of file size and loss of definition can be adjusted when the image is saved. JPEG images are almost always lossy to some degree. Another feature of JPEG compression is that "interlacing" can be used, in which the data is compressed in multiple passes of progressively higher detail. This means that as an image (particularly a large one) loads, a low-definition preview is seen before a higher detail image loads.

JPEG compression is most appropriate for photographs, because the loss of definition is less noticeable in a continuous colour image without sharp transitions between colours. Where large areas of a plain colour or sharp lines appear, this usually results in compression artefacts visible to the naked eye. For this reason, JPEG format is not suitable for most vector graphics or similar images.

PNGs

PNG (usually pronounced "ping") stands for Portable Network Graphics. The format was developed as an alternative to GIF, and although GIF is still very popular and widespread on the internet, PNG continues to gain popularity.

Unlike GIFs, PNGs support 24-bit colour. PNG images use a non-patented lossless compression method, so images will always appear crisp. Gradient transparency can also be used (using alpha channels), as can interlacing. However, PNG does not support other colour spaces such as CMYK.

PNG does not itself support animation, but a related format, MNG (Multiple-image Network Graphics), was designed to allow animation. However, MNG format is not widely supported because it is a far more complex file type. Its different file signature renders it incompatible with PNG viewers. APNG, a simpler alternative to MNG, has been proposed by the Mozilla Foundation, and is supported in development versions of Firefox 3.0. Several other alternatives are under discussion.

PNG file sizes are about the same as GIFs, but because they use more colours, larger file sizes are possible because there is simply more information to store. In fact, on identical images, PNG can often achieve greater compression than GIF. PNGs are a good choice for vector graphics which have both continuous colour and sharp lines, because they support the full range of colours and do not produce compression artefacts.

PNG images are widely supported, but not as widely as GIF images. Until Internet Explorer 7 was released, the Microsoft web browser did not support alpha channel transparency (although it did support the use of a single colour per palette as transparent). However, IE7, Firefox, Safari and Opera are all fully PNG-compatible.

By Brian Jackson

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