To uninstall, yes, you have to quit it first, either with Activity Monitor or the terminal (killall OpaqueMenuBar).
Then, you can delete the application.
To reset the desktop image, just open up the system preferences and go to Desktop/ScreenSaver settings. There, re-select your desktop image. Then it will be reset.
Sorry for any inconvenience caused.
If there's anything else, please, don't hesitate to mail me at firstname.lastname@example.org - I check it way more frequently than the comments here ;)