I didn't like them, so I didn't bother saving them. Sorry I can't help you with that, but you might might find these threads interesting/helpful:
Here's the image that I use for the standby screen, when the e-reader is in sleep mode, plus my preferred settings:
Home > Settings > System Management > Standby Screen > Show picture On > Show message Off
I first came across this image when someone here at MR posted it. I'm afraid I don't remember who it was.
If you are unsure where files are located, then you might find this helpful: