http:\\openhabian:8080 isn’t a URL that exists anywhere except on your LAN. Try this experiment, open that URL on your phone when not connected to your LAN.
Use a relative path and that first part of the URL that doesn’t exist anywhere but on your LAN will be replaced with what ever base URL you’ve used to access MainUI in the first place.
I thought this what is shown in a cloud is being received directly from my server within secure tunneling communication. Is there any possibility to display pictures I see on my device in a cloud?
Alternatively i could store pictures somewhere on the net. Right?
It is. But you’ve bypassed that functionality by using the full URL.
When you are on your LAN, the URL to that image is http://openhabian:8080/static/RaspberryPi_Logo.png. When connecting through myopenhab.org that same image’s URL is https://myopenhab.org/home/static/RaspberryPi_Logo.png. (NOTE your slashes are backwards in the URL but that doesn’t appear to matter since it works on your LAN at all).
Yes, as I said and is in the docs, use a relative URL path instead of the full URL. /static/RaspberryPi_Logo.png