JPEG or GIF?

a. JPEGs are better for Photographs and images with colors blending into other colors. They also permit a greater degree of compression than GIFs and can be smaller.

b. GIFs are better for images with flat fields of color.

GIFs can have transparency (like the ones below) and JPEGs can't.

GIFs can be animated.

PNG

People just don't use PNG as much as GIFs because of browser incompatability. only later versions of IE and Netscape support PNG. Gifs can do transparencies too, so most people just use GIFs, even if PNG's may be higher quality images

Something else new: SVG (Scalable Vector Graphics): they use XML, code is used to draw vector images it is written right into your HTML and may one day replace or challeng the use of Flash vector graphics.

And also: SMIL (Synchronized Multimedia Interface language): A mark-up language like HTML that will make motion graphics and rollovers, coming soon.