Add "last book" screensaver for Kobo

Epub only.
If someone knows how to get an jpg/png from a pdf on linux (e.g. convert by imangemagick? But i am not a linux specialist and have no idea how to get that one into koreader) I can add something for linux too.
cbz/cbr should be possible too, but I have not looked into it yet.

Do I see it right that the Kindle screensavers are handled by Kindle itself?
But on Kindle you also profit, you can see picture thumbnails now in the search results (long tap on found book)