How to add image to file format .mp3 on a Mac?
There are several places you can do this:

within iTunes. Simply right click, select Get Info, then select the artwork tab and select “Add Artwork”

from Get Info (Finder). Right (Option) click the file and select Get Info. Choose Artwork, then select Add Artwork. Select the art work you would like to embed.

in VLC. Go to Media Information screen CommandI  which will bring up a window of all the tags including artwork. Simply drag your artwork to add it.

via a tag editor like Music Brainz Picard. You can easily add the Artwork by dragging it to the artwork “field” in the editor and save it.

command line via ffmpeg
ffmpeg -i musicfile.mp3 -i coverart.png -c copy -metadata:s:v title=&quot;Album Cover&quot;-metadata:s:v comment=&quot;Album Art &quot; musicfile.mp3