That doesn't solve anything at all. The incorrect album art will still appear in the playlist and in the Now Playing window if the user decides to use "Album Art" instead of a visualization...
My advice is to simply download and add proper album art to each album, that's what album art is for.
If you want to completely delete the album art, first you need to find the folder where it is located, then click Organize -> Folder and search options -> click the View tab and uncheck "Hide protected operating system files" -> click Yes, then Apply, and OK.
You should now see the hidden album art files, simply delete them and you're done. Make sure that WMP12 is closed before you do this, so that when you finish, you can start WMP12 and (hopefully) after it automatically updates your library, the album art should be gone for good.
Also, in case you do not want this to happen in the future, you can prevent WMP from automatically downloading album info by clicking in WMP: Organize -> Options -> and on the Library tab uncheck "Retrieve media information from the Internet" -> click Apply, and OK.
Hope this solves the problem.