(I don't advocate importing PowerPoint slides into other programs - I've seen too many problems with that although some may have found a working model. I find it's better to let those speaker presentation programs do their jobs and let the song presentation software do it's job. The philosophical approach to song presentation is usually quite different than screen support for a speaker.)
What I usually do is to create a theme graphic that I use as the desktop screen image as well as in ProPresenter and Keynote/PowerPoint. That way, as I switch from one application to the next, or just have nothing displayed, the theme graphic appears on screen. The theme graphic should follow the theme of the message or series for continuity. Even with that theme background, I may still use certain pictures and backgrounds during the worship singing but only for punctuated effect and return to the theme graphic so that connection is never lost.