put a water mark on a report diagonal over the header, detail and footer
No. But this might work--
Copy the report naming the new one 'sub.
In sub delete everything but the detail.
In the main put the watermark image over the entire header and one over the entire footer.
In the main delete the entire detail section.
In main, add the sub as a sub report to the header section, linking appropriately.
In main add the watermark image over the entire subreport.
You may have to adjust the Can Grow attribute of the sub report and it's watermark