The path must be incorrect... or else it would work.
Maybe it should be:
<img src="/images/art/CelticMandala.jpg" alt="" id="WADAPageHeaderImage">
Is your page in a subdirectory? Is the image folder in a different directory?
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.