Background Images for Version 7.0 (Portal)
From OnePager Documentation
About Background Images
Images in the background are an excellent way of decorating the chart with a company logo, a picture of the product the project is building, or to show other relevant visuals. As an example, here is a chart with a background image showing in the lower left corner of the graph:

The following links provide detailed articles on inserting, moving/resizing, editing, and deleting background images:
Inserting Background Images |
Moving and Resizing Images |
Adjusting Image Transparency |
Deleting, Ordering, and Changing Images |
(20.0.1-70)