I’ve been trying to work out how to nicely fade images into slides in Keynote, like in this example. Unfortunately, there’s no simple way to do it with just Keynote. You can add masks to an image to crop the visible area, but you can’t fade the edges.
One workable solution I’ve found after some experimentation (i.e. hours of cursing) is to use The Gimp to doctor the images before I add them to Keynote. I found a tutorial on making a faded frame with quickmask which does the trick, though it misses out one all-important step: if the image you load does not already have one, you need to add an alpha channel before the transparency will work.
So, now everyone knows my GetTogether slides will have some faded images…. 
Related posts:
Your problem here is forgetting a unix rule that has something to do with having lots of small tools that do one thing well.
Keynote is not an image manipulator, and (in most cases) The Gimp is not useful as a presentation engine.
Personally I would never have even attempted to do image manipulation of this sort without firing up Photoshop..