As a general rule your site should not contain large images, or too
many of them, unless they are explicitly linked to as such. (they are slow to
download with slow connectivity)
Make sure you keep a copy of the original loss-less images and
resize them and compress them as you see fit.
All images must have "alt" attributes (or else blind and other disabled people
cannot understand them).
Using images as links when text can do,
is considered a bad idea (since the text will load much faster).
If you want to have a navigation image, include some textual links below it.
Animated images are generally bad if they appear at primary pages of the site.
Make sure your ads, if you have any, are non-animated.