https://musicbrainz.org/edit/22912203 for example adds a picture without any type, but I wasn't sure if

• the type was not set
• a type was set but add cover art edits don't display it
• a type was set but not displayed due to some bug

Only after looking at other add cover art edits and the release itself was I able to deduce that "the type was not set" it the correct answer.