Go to the dashboard and click on Pages and Themes and then Attributes.

At the bottom, select Image/File and click on Go.

In the next screen, enter background for Handle and Background Picture in Name. The handle is what you'll need to access your attribute from code.

Click on Add Attribute.

Go to Page Types.

Click on Edit next to the first page type.

In the middle, check the checkbox next to Background Picture. This makes sure the attribute is displayed by default for each page of this type. You can add attributes to a page, even if this checkbox isn't checked, but it saves one click.

Click on Update Page Type.

Do the same for all page types where you'd like to use this attribute.

Go to the home page and enable the edit mode.

Click on Properties and select the last tab Custom Attributes. This is where you can find all the attributes on a page.

Use the file selector next to our new attribute to select a new background...