Many users may choose not to load images, so it is extremely important to place meaningful alternate text tags in each of your embedded images. When creating alt tags, keep in mind that you are trying to convey the message of the picture, not describe it. Therefore, putting "Yellow button" as an alt tag for a yellow button is not nearly as effective as using a "*" instead.

A.J. Flavell has written a very thorough document on writing meaningful alt tags available at:

http://ppewww.ph.gla.ac.uk/~flavell/alt/alt-text.html