Did this happen when you upgraded? If so you have several albums with cover Art stored in the MC26 folders. To rectify you need to edit the tags of the affected ablums. Several ways to do this.
In album view, right click on an album and choose cover art, then a submenu with many ooptions including get from internet, or if you want to go back to your previous ones, choose from file then navigate to %appdata% then JRiver, Media Center 26, Cover Art to pick up your old one.
Another way is within the Tags panel on the left, scroll to the image. Sometime one track will have the image while the others don't. Clciking on the image (not right-click) gives you similar options as above. Including copy past from clipboard where you can copy paste the same image among tracks.
This issue occurs if you have used inconsistent ways you have applied them. It's not really a MC thing it s a process thing when files entr your library from all different sources. You have to keep on top ones that are wrong and fix them. I have "Image File" listed as one of my filters in the Audio Files view so I can keep these clean. Any item in my files that is not "Inside File" or Folder.jpg, shows you which ones you need to fix up as above, because they will be stored in the AppData folders. Hope this helps.