Microsoft PowerPoint backgrounds can be problematic. You don't want them to distract, but no background at all can be harsh or boring. (Harsh can be an appropriate look, but never settle for boring.) If your focus is a picture, try using the picture itself as the background and the focus. A sample PowerPoint file is provided to help illustrate ths tip.
This may require some patience, since in the slide focus the third slide requires a series of several steps. But it's always interesting to find an alternative to what we know, when it comes to Photoshop.