Exchange image during mouse hover / click

Exchange image during mouse hover / click

[David Reitter]

I'm dealing with images included in a buffer, which implement buttons  
for the user to click on.

I'd like to display a slightly darkened image when the user hovers  
with the mouse over the images, and an even more darkened image when  
they click on it (during mouse-down).  This gives good visual feedback  
and improves the UI.

Question is: how do I do it?   While the tool-bar provides image  
vectors to do just that, I can't seem to find functionality to  
implement this.

The `mouse-face' property of faces only allows me to specify another  
face, so that's not useful to me.  There is no way to darken anything  
displayed via a face property, and I wouldn't really want to do that,  
because I'm suppling the actual image.

Is this possible now in Emacs 22?  If not, could it be provided in 23?