Lol, you can't access that over regular USB connection
Your options are:
1) if you're using KOreader use its built-in File Manager to browse Kindle's file system (you'll maybe need to change the settings to make it display unsupported file types?).
Then you can copy the png file from that location into "/mnt/us" ( = what you see on your computer via USB cable).
2) install and use "kterm" if you familiar with using terminal / "command line" commands to navigate and copy files
3) install USBNetwork hack and setup SSH connection over USB cable and do the command line staff that way
4) instead of access files directly from your Kindle alternative approach would be downloading firmware bin file from Amazon's site > use "kindletool" to unpack that bin installer > open the roofs img file in something like 7zip and then go to that stated folder location inside the img/archive file
5) have someone else extract and send all screensaver images to your.
Option #1 is probably the easiest method for average Kindle user