Given that the Pandora screen is 800x480 and quite some room is needed for the function descriptions, it should be something like 500x300 for the keyboard image or so. A PNG with alpha would be best, with a background that is mostly transparent and the keys that are mostly opaque. But if that is too hard to do, I would also be happy with something that looks good on a solid black background, since that's probably what I would use anyway (I might show the moving menu background instead, but that will probably make things too hard to read).
It's not that it would be difficult to implement such graphics, but if they're not deon proprely then you can end up with either inconsistency if the goal was strict consistency, or something that looks alien if the goal was to have something that extends (as in class inheritance, but obviously more abstract since it's with graphics) the way the software looks, feels, and works.
The way I'm approaching things with guihints is to provide something that results in an elegant solution and is readily usable. It follows that providing a generic keyboard overview screen is not something I would do unless I could make sure that this screen would fulfill the superordinate purpose of guihints (which, as you may remember, is to hopefully improve overall ease of use by introducing a particular kind of familiarity across software).
What I had already planned on doing though is to offer, for a limited time at least, help with integrating guihints into individual projects, so that I can learn about things I could improve further. So, what I would do is provide you with a keyboard screen just for Microbes, and then depending on how that evolves, maybe include a generic-ized revision of it in a guihints package later on. Note though that I can not do this _RIGHT_NOW_.
Just PM me if you are interested (the same goes for any other developer looking for any kind of advice on guihints usage).