This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
You can use your own background-images of course.
Create a folder for your background-images like
Upload your image or your images.
Configure the user-interface (Constant Editor):
- [START - CUSTOMER - CSS - BACKGROUND] > file path: fileadmin/my-background-images
- [START - CUSTOMER - CSS - BACKGROUND] > foto theme: [empty]
- [START - CUSTOMER - Config] > lockFilePath: fileadmin