amicka: You probably found this on your own, but I'll post the details for others who may be interested:
On the Settings|Diagram screen (where you selected the background image) you can specify if the image will appear on the display and/or the printed reports.
The option to frame the background image determines if the image size is considered when the software determines the overall extents of the project. Otherwise, the extents are determined only by the existing nodes, and the background image is allowed to "overflow" off the edges or the screeen or page.