the best thing is always having a cover.jpg in the same folder as the flac of the album
that way if the cover is not embedded, Audirvana will automatically choose that file for cover.
But to be sure that your things always follow, click that album in Audirvana, that will show you all the songs, then click the little tag icon in the right bar of window… and change the cover… select your image in the folder, then save… you will see that in the window of the finder that all your flac files changed in date… the cover is in them now. Have fun
it is not because you downloaded something from a pro company that things are done well or at your taste