I try to avoid Calibre because I find its UI to be aesthetically offensive and sometimes outright confounding.
So I’m late to find out about this plug-in. It will allow Sony Reader, PocketBook, and Kindle owners to use eBook covers as screensavers. A full discussion of it is here.
I found out about this only because the author just modified it to work with the Nook Touch too. That discussion is here.
Note that the new Sony Reader WiFi is supposed to have this capability built-in, so Calibre might not be needed for it.
And yes, I know the precise term for this is the standby screen, but screensaver has been in use for so long, it’s what people usually call them.