Images
Some people use text-based browsers. And some people turn off the
automatic image-loading on their graphical browsers in order to speed
up download times. What does your page look like if the images don't load?
Image Intensive Pages
These take forever to load. Have pity on the poor person with the slow
modem half the world away and keep the images to a minimum. At the
very least, warn your visitors that they are about to download many
kilo- or mega-bytes of images.
Image Maps
Image maps should be your last refuge. If there is an alternative to
an image map, use it!
Images Used for Navigation
Always use text-based links for navigation for the reasons given above.
Animated Images
They used to look very clever but flittering, bouncing, blinking images
tend to distract the eye and fool the browser into thinking it has not
completed loading the page. If the purpose of the page is to show off
the animated image, fine. Otherwise, they are best avoided.