ffmpeg: add album art with fluent-mmpeg

Question

Currently I am using fluent-ffmpeg to convert a video into an mp3 file and add metadata.

I am adding the title and artist like this:

.withAudioCodec('libmp3lame')
.toFormat('mp3')
.outputOptions('-id3v2_version', '4')
.outputOptions('-metadata', 'title=' + songTitle)
.outputOptions('-metadata', 'artist=' + songArtist)

I would also like to add album art. I am looking to do something like one of these options:

.outputOptions('-metadata', 'title=' + songTitle)
.outputOptions('-metadata', 'artist=' + songArtist)
.outputOptions('-metadata', 'albumArt=' + base64image)

or

.outputOptions('-metadata', 'title=' + songTitle)
.outputOptions('-metadata', 'artist=' + songArtist)
.outputOptions('-metadata', 'albumArt=' + pathToImage)

How do I add the album art to the mp3 file?