All my composer images are in c:\users\mike\appdata\roaming\j river\media center 27\cover art\composers
The file names do match the composer names - eg Bach, Johann Sebastian (1685-1750).jpg - and the images show up as expected in JRemote2 and MO 4Media. Only a few show up in Media Center itself, and those that do show up don't have the composer's birth-death year in the composer name. The brackets are probably causing MC some grief, although it still knows how to deliver the images to the remote apps. And MC did display the composers at some point, otherwise I wouldn't have 75 files in the composers folder.
It looks like I started adding composer images on December 14 of 2019 while using MC 26, and stopped adding them on January 6 of 2020. I'm not sure if that's because they stopped working, or if I had added all the composer images I needed.