In order to use an image at poster-background sizes, you have to have a very high resolution image. OOo provides some general-purpose images at resolutions that are useful for typical page sizes. For something the size of a poster, you'll have to find your own high-resolution image, or just use a solid color or gradient for the background.
In fact, you may be better off designing for a white background (i.e. no background), and then print the poster on colored stock. The print shop should be able to advise you which is the better way to go.
PS: The other thing is, you don't want to export your poster design as an image, and then send that to the printer. Export your design as a PDF, and send that to the printer. In fact, if you do that, you may get away with your current background image, depending on exactly how you're using it.
AOO 4 • Linux • Fedora 17