Custom images as overlay levels or background

There should be a possibility to upload a set of images that can be assigned to replace a given overlay level, or to be treated as a background on a given overlay level.

Correction - not "overlay" but "zoom" level.

changed milestone to %v13.1.0~beta.0

created merge request !768 (merged) to address this issue

mentioned in merge request !768 (merged)

closed via merge request !768 (merged)

mentioned in commit 3fb09925