There are two types of images used on web pages today. The first is JPEG, or JPG. This format is best used for photographic images since it can display millions of colors. The other format is GIF. GIFs can display up to 256 colors, and are best used for graphics instead of photos.
Aside from the standard GIF, there are two other types of GIF you might use on the web. The first is a GIF with a transparent background. This image type allows you to have an image that sits cleanly against the background without "white space" showing the rectangular nature of the image. Programs like PhotoShop make it fairly simple to create a transparent GIF, and I suspect there are freeware or shareware programs that might help you do this as well (a few years back there was a program called LviewPro that did this, but I am unable to find current information on that program today).
The other type of GIF is the animated GIF. If you've been to a web site with one of those banner ads at the top or bottom, you've most likely seen an animated GIF. There are two types of animated GIFs: repeating and non-repeating. Most animated GIFs are repeating, which means that they never stop "moving". But it is possible to animate an image for just one "loop" so that it will stop "moving" once the loop is completed.
If you want to find out how to create your own animated GIFs, you might visit BUILDER.COM's article, Get Animated!. You also might want to visit Yahoo's index of Aninmated GIF collections to see if someone is sharing an animated GIF that will meet your needs.
I used animated GIFs about a year ago, but they seem to be on the verge becoming as annoying to some people as blinking text. They can take a long time to download and seem to add little to a web page.
A final note: a new image format, PNG (Portable Network Graphics) has been created, and is expected to become popular as newer versions of Netscape and IE begin to support the format. You can from the W3C.