1. Open the File Manager

2. Navigate to /server/docker/cfg/
3. Create a file called server.cfg

4. Right click the file and select Edit

5. Add this line with a link to your image. You can use a website like this to a get a URL to your image. Your image needs to be 512x256 or 1024x512 and in either .jpg or .png format.
server.headerimage: "https://myimagelinkhere.png"

6. Restart your server and the banner will appear in game
