GIF : Compuserve's semi-proprietary image format.
Images are discretized and dithered down to 256 unique colors,
and saved as indexed color. Transparency is supported in GIF 89a.
JPEG : JPEG stands for Joint Picture Expert Group,
and is the culmination of several man-years of work in image
compression. It is based on the discrete cosine transform (DCT)
and is excellent for photorealistic images. Transparency is not
The PNG image format seems like an ideal
image format for web images. It supports a variable level of
compression (including lossless) and even has a built in alpha channel.
only NCSA Mosaic supports this image format, and (not surprisingly)
doesn't support the
alpha channel (alpha compositing is actually pretty slow to do in
Don't expect to see this feature for a couple of years).
TIFF, PICT, etc. :
These formats aren't supported by web browsers because the don't
support lossy* compression.