First download an install Featured Image Module on your Joomla.
Creating a new Module give yout the view below:
Now you have two options:
1. Upload a Background-Image (or color) in options Tab and Save Module. Afterwards the layoutbuilder shows your image and you can start to add elements.
2. Use a pre definied design from Designs tab and save it. Afterwards you can edit the elements.
General Workflow: