If an actors thumbnail has already been cached Kodi can be a bit reluctant to update the image. If all else fails you can try replacing the cached image directly with the new image, but take care, this is a low level mod.
Actor thumbnails are stored in the user directory .kodi/userdata/Thumbnails using a hashing system. You can determine the hashed value of the image by looking in the Textures database stored in the directory .kodi/userdata/Database.
I’ve used DB Browser for SQLite to look into the databases on my windows machine.
If you look in the textures table you should be able to relate your video’s/actor’s url to the hash of the stored thumbnail. The image above shows that the file 2002-02-03 S01E01-thumb.jpg is cached as the file a5605273.jpg. Replacing the a5605273.jpg file with the new image should lead to the new image being displayed. Don’t forget to update the source image though. At some point you will no doubt clean and reload the full database at which point the source file will be loaded.