Both PowerPoint and Impress slides are specified in inches or cm, rather than pixels, and these real-world units are somewhat arbitrary given that the presentations are normally scaled proportionally to fit whatever screen they're shown on. (Or non-proportionally, if somewhat's got the screen settings wrong)
I'd go with an image that's big enough in pixels for any screen you're likely to show it on; just scale it proportionally to fill the background.
But for best results try to match the proportions of the presentation itself to the screen you're going to use. Both PowerPoint (at least up to 2003) and Impress (latest version in mid 2011) "sized for on-screen show" slides defaulted to a 4:3 ratio, which is too tall for modern widescreens. In PowerPoint 2010 (and perhaps 2007?) you have the choice of 4:3, 16:9, 16:10.