Your embedded image (the one in the audio file won’t work in Volumio as far as I can tell.
You are correct to put the album cover image in the same directory as the audio files
Keep the image under 1 MB in file size, make sure it’s named folder.jpg or cover.jpg
(you can also use png I believe).
I size mine to 500 pixels x 500 pixels, but I know larger (at least up to 1400 pixels works as long as the file size is under 1 MB.
You might need to refresh the folder, and possibly refresh/update the music collection under settings, there’s also an image cache refresh function in settings.