M4P and M4A - AAC Audio Files
File Juicer can extract artwork from M4P, M4A and MP3 files. It can also extract a small amount of text, similar to ID3 tags in MP3 files -- including your name if you purchased the file from the iTunes Store before Apple removed DRM in 2009.
M4P files are DRM-protected AAC audio purchased from the iTunes Store before 2009. Apple dropped the DRM from most music at that point, so M4P files are now a legacy format. Apple recommends backing up your library to protect these files.
M4A files are AAC audio with cover art and other metadata tags. Plain AAC files without tags contain no artwork for File Juicer to extract.
Using your Music library with Roon
If you are importing music from Apple Music into Roon and have more than one image per album, you will normally only get one image across. Roon can handle extra images if you place them in the same folder as the audio files.
AAC - Advanced Audio Coding
AAC is an ISO standard and the default audio format used by Apple. Wikipedia has more detail on AAC.