Okay, I just did a little experiment because I had some of my personal screensaver images with the little on battery on them as well. I do not have any of my folders hidden and I don't see one called "system".
I opened the folder that contained the original images I used for my screensavers and deleted both of the screensaver.manga files. Then I re-saved everything from scratch (17 images). If there is a way to remove just one or two, I haven't been able to figure out how to do it.