You may also consider a pre-developed tool, Bobby, which you can download and test your entire site for more than just the ALT tag problem. Bobby's been sites many times with Section 508 regulations for government web sites in order to improve accessiblity and is a well-trusted tool by others.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important