Hi Aggelos
Thank you for your message and great to hear that you are using
https://www.map.army.
We are currently working on an improvement of Vector- and Image-Overlays. This will include the option to add overlays from online sources and include them into shared projects. The new version of
https://www.map.army will be online by the end of this week.
Regarding your question:The json file is required in combination with the image overlay. So, in total you will have two files:
Follow these steps to create an Image-Overlay in https://www.map.army:- Select Images Folder (please use an empty folder):

- Place the Image (your map in the PNG format) in the folder selected in step 1.
- Select the image and place it onto the map:

To load an existing Image-Overlay follow these steps:
Please do not hesitate to contact us should you have further questions on
https://www.map.army.
Kind regards
Martin from the map.army team