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