*edit* Arowana's answer below is a much more simple fix to this question.

I'll leave this here in case anyone happens to find it useful though. (minus the image examples because I think it's pretty straightforward.) This method is probably better suited to people that have oddly shaped buttons/hover areas with transparency. */edit*
If you're not dead-set on imagemaps, then this is really easy to do with imagebuttons. ^^ Hopefully this example is clear because I'm pretty much copy-pasting from a PM I sent a while ago.
Open your imagebuttons in whatever image program you use. Then add a new layer underneath whichever button you are currently editing. Make sure the area you want hovered is completely white on that new layer. Then go to the opacity for the new layer and turn it all the way down to 1. (Or even less, like .5, if you really want to make sure it doesn't show up. As long as it's a higher number than 0 )
Then merge your layers and save and tada! The player can now hover over the button in the area you put white under even though the white isn't visible. (You might want to save it in a different file just so you have the original somewhere if you decide to do something different. Or have the original copied somewhere else.) If you do this and have problems with it though, let me know. It's been a while since I've worked with GUI now.
It's also assumed that you have focus-mask set to True.
As for the use of imagebuttons in coding if you don't already know:
http://lemmasoft.renai.us/forums/viewto ... 51&t=22565