There's ghosting everywhere, and we can't do a thing about it, that's how the e-Ink screen works
.
It's more noticeable on some patterns, and depending on what was previously on the screen, so that may explain why you didn't notice it with default screensavers, but it's there, regardless of your pictures.
That's why the Kindle 3 does a flash to *white* before showing a screensaver, it helps a bit with the ghosting.