Here are some pretty nice Halloween screensavers I came across the other day:
http://booksprung.com/halloween-scre...or-your-kindle.
I mostly use my own images as well though. If you have even basic Photoshop skills, it's pretty straightforward: resize, downsample if you feel like it, convert to greyscale, adjust brightness/etc, upload, restart Kindle. And that's it, really, in a nutshell.