1) You don't need USBnetwork.
2) See the information here:
http://www.mobileread.com/forums/sho...d.php?t=195474
If you successfully installed the jailbreak, this shouldn't be a problem for you.
In the post to which I linked, there are two attached files. If you only wish to use your own custom images as screensavers, you only need to install
kindle-linkss-0.7.N.zip.
If you want the cover of the last open book to be used as the current screensaver, then both files need to be installed one at a time. Just follow the instructions in that post. If there is something you don't understand, ask away.