[Plugin: Rotating Posts] Resetting Defaults in this plugin

Hi!
I’m a total minger who had this plugin running really well on my site, but then I got it into my thick head to mess with the thumbnails and a couple of other things….
Suffice to say, now it’s broken and I can’t fix it.
I’ve tried uninstalling and then reinstalling the software but all my bad settings came back with the re-install.
Could someone please help? Instructions for (major) dummies would be much appreciated!
Thanks.