There are a few ways to put your own custom picture in the background.
Replace background with “Banner Image”
Override Site Banner
In RW7 you can replace the current background.
Just drag your image.
Edit background with Image editor
You can edit the background with any external image editor (Photoshop, Gimp, Pixelmator…).
Open the Theme content and all images ( contents/images/editable_images/image_…jpg).