I took a look around the different sites that I use and most of them run into the same problem, a watermark doesn't work with charts (bar/map/whatever).

On an MSDN blog they mentioned this approach which might work. Create a background with the image and make all the objects in the report as having a transparent background. I tried a simple chart on my dev system with this approach and it worked nicely.

http://blogs.msdn.com/b/emeadaxsupport/archive/2009/04/30/watermarks-on-reports.aspx