Larry,
When we used PowerPoint for all our projection needs, we did the image per song method. But I quickly got tired of reworking themes and trying to maintain a theme library that big. So I switched to just selecting an image that goes along with the theme of the service for that Sunday. Like for Mother's Day, I choose a nice floral BG, for Easter I choose an image looking from the Grave out, then for Memorial Day I choose a nice Flag BG (granted I really wanted to use a Video BG with a subtle waving flag, but Video BG are not working right on our Church PC at the moment).
While OpenLP does have an Image and Media Libraries, and I know you can replace a service items image from them. But from what I understand, that only works after the item has gone live. I don't want to have to switch BG's in the middle of projecting it.
It would be nice to be able to use the Image and Media libraries as BG picking points. I Like having an Over arcing Service Theme, that's useful for items that don't really need their own images. But it would be nice to just pick images and video BG's from a Media Library, sort of like how Worship Tools Presenter does.