Map editor: use custom image not working

[krabina]

Setup

• Maps version: 7.4.1
• MW version:1.31
• PHP version: 7
• SMW version (if applicable):

Issue

The maps editor has a feature to display custom images, but it is not working. Nothing happens when you click on "Ajouter la couverture d'image" on sandbox: https://sandbox.semantic-mediawiki.org/wiki/Sp%C3%A9cial:MapEditor

This is probably not so much a problem of the map editor, but I guess the feature has been lost along the way. So in general it would be great if we could use images as maps (e. g. historical maps) that are overlayed or anyhow used with the maps extension.

[JeroenDeDauw]

The MapEditor is not maintained by me and has several unresolved issues here on the issue tracker.