Huh. That indeed sounds strange. The only thing I can add is that the screesavers thingy is way more fragile on FW 5.x than before:
Anything that the framework doesn't parse properly will wreak havoc, and basically 'break' the screensavers 'til the next reboot (what you're experiencing: it's showing nothing, just the last thing on screen).
So, check you files again, and make sure there's absolutely *nothing* extra in the folder (the hack should take care of that, mostly).
To really pinpoint if your files are the culprit, you could, as twobob said, try with a random set of images built by somebody else (unfortunately, that's not as common for the PW as it was in the olden times of the K3). If there's nothing fishy copyright-wise in your images, you're also welcome to attach them here, and I'll try to figure out why they might upset the Kindle framework...