3 Good reasons to use image 'ALT' tags !!

1. Blind people surf the web (yes its true). They obviously cannot SEE the images but their electronic readers use the ALT tag to hopefully verbally describe what the image actually is (ALT stands for ALternative Text).
All you need to write is some thing like <img src="whatever" alt="descriptive text here">

Here's one I prepared earlier - run your mouse over the image .... .

2. The effect you see in (1) is also visually helpful to fully sighted viewers and can enhance your page by giving the text effect a bit like a javascript mouseover.

3. If you have a big page that loads slowly and you don't have any HTML ALT tabs on the images then the viewer doesn't actually know if there are any images to come after the main text has been loaded.
But if you have an alt tag in your HTML then the viewer can either chose to wait for the image to load (after reading your ALT description) or they can move on to the rest of the site.

If you visit my site page about 'accessible web design' you can follow the links to the RNIB (Royal National Institute for the Blind) for even more information on making your site more viewable for blind people (merely adding a few extra tags here and there can make all the difference and vastly improving the surfing enjoyment of Blind and partially sited people - and it costs nothing at all except a little extra html coding time.

For your info - The normal way to put an alt on a spacer image or a purely decorative image is alt="*" or alt=""

Read also Sucky or Savvy for another authors viewpoint.