I've added a jpg file to the App_localResources folder and in the document properites specified the photo in the Background propery. In the designer it shows up as the background but when i run the page i still get the white page background.
Guessing that perhaps you have a stylesheet which might be overriding the background?
I would check if the built in web server was still running and stop it, then re-run your application.
* Right click on the tray icon and select "Stop".
The page is being cached, you should see your changes now.
There's no distinctly ASP.Net way of doing this.
The canonical HTML way is to include this CSS:
body { background-image: url('background.jpg'); }
If you defined the body tag with runat="server"
you could add the style inline using the Attributes property, but this wouldn't be a good idea. Layout details like this should go in the Stylesheet.
I used fiddler to trace the calls on the image. App_LocalResources\*.jpg return an 403 error. The App_LocalResources folder is really for use for localization. If I move the image into an image folder it works fine. Also rather than setting the Background property, use the Style property and the background property there.