Since a cover art image doesn't contain song or album metadata, TuneUp has no way of checking if your current cover art is correct or not. Additionally, the iTunes Application Programming Interface (API) does not allow 3rd party applications like TuneUp to overwrite existing cover art.
If you'd like to delete your current incorrect artwork and find new cover art through TuneUp, please follow the instructions below:
1.) Highlight all the songs for which you want to delete cover art in iTunes
2.) Press Control-I (or Command-I if you're on a Mac), and click the Album Art checkbox
3.) Press the "Ok" button at the bottom of the window to save your changes