If you want to use your own image for a background you can add it to the Image Selection dialog.
When you are in the Image Selection dialog click on the "Add image" button:
In the Open File dialog navigate to the image and click "Open":
To make it easier for you to find and use the image, assign categories for the image by checking the appropriate boxes. If you want to add a new category click "Add new category".
After clicking OK your image will be accessibly through the Image Selection dialog immediately.
To remove an image from the Image Selection dialog: