Yes they were stored in the pictures folder local drive...
I don't know if it would fix the problem, but I suspect it might, if you stored the image under the default Cover Art directory, as per below.
When I added the Logos using the Cover Art "Add from file" functionality. They were originally stored in a separate location, but when I selected the file, MC asked if I wanted to save a copy in my default Cover Art location, as set in Options. I clicked yes for all the Cover Art. MC then created a sub-directory of the Cover Art directory, and called it "Files".
So my Logos are now stored on the Server in the directory;
C:\Users\htpc\AppData\Roaming\J River\Media Center 21\Cover Art\Files\
It also renamed the files to match the Database Key of the TV Channel. So an example logo file for my Channel "1 ONE", with a Database Key of 403925 is;
C:\Users\htpc\AppData\Roaming\J River\Media Center 21\Cover Art\Files\403925.jpg
My default Cover Art directory on the Server was only used for Series and Season Cover Art, as for Audio I use the setting of "In the same folder as the file", and I also have ticked the setting "Also store image in the file's tag". Now the Cover Art directory is also used for Channel Logos.
I suspect if you did the same, the logos would have re-appeared in MC.
I also suspect that if you moved the Channel Logo files to the equivalent of the above location, renamed them as above, then selected the channel in a Smartlist that lists the channels, and ran "Right Click\Cover Art\Quick Find In File / Cover Art Directory", that all you Logos would re-appear in MC. Try it with one channel, and if it works, you can re-add all the Logos at once by selecting all channels and running that command. That may be easier than going through each channel one at a time and re-adding the Logo Cover Art, and if anything happens like this in the future, all Channel Logos can be re-added with one command.
PS: I know if the "Image File" tag points to the correct file, and the file exists in that location, it should just work anyway. But I have found that when using that method I sometimes have to "activate" the setting by clicking on a TV Show, Movie, or Series Name. By naming the Cover Art (Logo) file the same as the file it belongs to (Channel Database Key), and putting it under the default Cover Art location, MC just seems to show the image automatically, without the need for a "refresh". I assume it would do this after a Windows reinstall, if everything was put back where it belonged.
I'm having logo transparency issues over the last 2-3 releases (I think 21.0.27 was the only version since the logos were implemented that respected transparencies properly for me).
I don't think any release I have had Logos in has respected transparency in the Logo background.
EDIT: Actually, that is because all my logos are JPEGs, and that format doesn't support transparency. Even if I add a PNG logo, then allow MC to move it to the Cover Art directory, MC converts the file to a JPEG. If I don't allow MC to move the file to the Cover Art directory, then MC uses the PNG file, but in both the Smartlist and the Guide, MC displays a checkerboard background rather than transparency, much like Photoshop displays a checkerboard background when editing a PNG with transparency. A GIF works the same way.
If I open a PNG or GIF with transparent background in MC21, for full screen display of the image, then the transparency is correctly displayed.
This is an area that could be improved. Both by not converting PNG files to JPEG when used for Cover Art, and in having MC correctly show transparency in Cover Art.
