Basically I've abandoned image maps as they don't allow me to highlight the part of the tree being selected. After some fussing I switched to using HTML DIVs, which sit on top of the image. This took a little while to get working, CSS and DIV placement drives me nuts. The trick is to give each DIV the style position:absolute;, and (and this is important) make sure that the DIV is written as <div ...></div>, not <div .../>.
The trees now show a pale blue highlight when you mouse over an area you can click on, and if the corresponding subtree has an internal node label, that label is also highlighted. In the same way, if you mouse over the region on the right that corresponds to a labelled internal node, botht he label and the subtree are highlighted. I think this helps make it clear what parts you are selecting, and gives you the option of selecting using a name, rather than clicking on part of the tree.
