We use background images to improve the visual appearance of our reports. The images used to resize based on the size of the browser window. However, recently the images stopped resizing proportionally to the other visuals. This occurs in the service and mobile views. It occurs in reports and dashboards.
To illustrate this issue, place an image on a report spanning the entire report. Add some visuals on top of the image. Make your browser window smaller. The visuals get a little smaller but the image gets tiny and the visuals no longer "fit" within the image.