Do you have a "TV Channels" smartlist? If not, you can create one. Playlists > Smartlists > Add Smartlist, use the following properties:
Under Rules: "Media Type" is "TV"
Under Modify Results: "Limit database to" "Main"
Now you can display the smartlist, and show "Image file" field in a column. If the "Image file" field is not empty for the channel you are having trouble with, try delete that image file. If that field is empty, then navigate to your Cover art folder (usually C:\Users\<user>\AppData\Roaming\J River\Media Center 24\Cover art\, but could be in your customized location), and open its "TV Logos" subfolder. You may find some files named in one of the following fashions:
<Channel Name>.jpg (or .png)
or
<Channel name> <nnnnnn>.jpg/png where <nnnnnn> is a number representing the channel's database key.
You can delete the images if they are not correct.
There is another place these logo files maybe stored if you used channel logos during the earlier stages of Channel Logo. Those images files are in the "Files" subfolder under "Cover art". The files are named in the form <nnnnnn>.jpg/png and therefore are harder to identify as you must go by a channel's database key. The database key of a channel can be found on "Manage TV Channels" window.