how to add album art to folders already containing flac files ?

New user to Touch and LMS.

I noticed that some of my music folder contain album art others do not. I would like to add album art to the folders that are missing the album art.

Can I just go ahead and add the album art inside each music folder that contains the flac files ?

What is the appropriate picture size ( jpg) and what is the naming convention? I notice that the names of the album art varies from folder.jpg to albumname.jpg

Is a rescan required after all the album art is added ?

Thanks