Is there a way to manually import a movie cover for you library if one does not come up when selecting "fix incorrect match"?
Not presently.
Devs said in some other threads (sorry, don't have links handy) that they are looking into adding another metadata source outside of Gracenotes (probably Guidebox--which is owned by Reelgood) and letting users manually edit their own metadata. So, not yet, but sometime in the near future.