Req: images for down-mouse and hover events

[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  
for general images.

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?