I'm probably gonna regret this....
You have to use the search in options window. This would help you find most, if not all, of the things I've mentioned. I used the words "this is what you need to do". Not what do you do today? I've been told that many times, and it really does not matter much. If you wish to solve this you have to do something about it. Not explain more.
You go to this options and change or do what I write. Start with the PC and library you normally download cover art from.
1. File - Library - Backup Library
2. Go to your Cover art directory and COPY those to another folder. Do not move them.
3. Options -> File Location - Cover Art
Audio mode: In a specific folder
Folder: "C:\Users\Joao Urbano\Pictures\a"
- Check the box for "Also store image in the file's tag
4. Right click after selecting all albums, and select "Quick find in File/Cover art directory"
5. If there is album covers missing, you'll have to paste the local images to the tracks, or right click and get thumbnail from the built in downloader.
Once all is tagged.... Do the following:
6. Options -> File Location - Cover Art
- Audio mode: In a spesific folder
- Folder: \\DISKSTATION\Cover Art\
You'll have to create the "Cover Art" directory on the share
7. Right click all the selected tracks and choose Cover Art - Save cover art to external location specified in options
8. In MC, select all your audio tracks and right click. Choose Library Tools and then
- Update tags (from Library) and then..
- Update library (from tags)
This will make SURE that the files are containing all the right tags, and that the library are updated from the file tags. I'm sure this is not necessary, but still...
Your one client should now be 100% working, as long as another client don't ** it up. It should be safe to delete the old cover art location, but make sure you keep a backup just to be sure. Also check that the new cover art directory on your NAS looks ok.
Now you need to change the setting on the other clients as well. I would have taken a backup of the settings in the main library and restored this backup with setting to the other clients. But you might have some different settings on them, which will cause more work for you. In that case, do the following.
9. ToolsOptions -> File Location - Cover Art
Audio mode: In a spesific folder
Folder: "\\DISKSTATION\Cover Art\"
- Check the box for "Also store image in the file's tag
10. In MC, select all your audio tracks and right click. Choose Library Tools - Update library (from tags)
11. Tools - Options- Tree & View - Thumbnails - Build missing thumbanils
In the case the old library is causing problems, and the libraries on the different clients should be identical, you could delete the library and import the files again. That would probably be the easiest.
If this instructions are not clear enough, I seriously need help from someone else. Because I can't spoon feed you more than this