I'd pick something like this:
But with a pure white background instead of light gray. The menu makes it identifiable as coming from PalmOS. Just a screen of smoothed text is nice, but is also very generic. Anyway, that's just my opinion.
At any rate, the image must be small to fit on the front page. I don't know the maximum allowed dimensions, but the current front page has images that are between 120 and 220 pixels wide.