Scale background images similar to the content
I want to use a background image that fills exactly the available background on my slides, no matter what screen size it is presented on (so the background image should scale as the content does).
For example I use 960x720 presentation size and I have a 960x720 pixel image. The options "cover" and "contain" don't provide the desired result since they will draw the image over the whole screen (rather than the slide). So when I select the only remaining option "original" it fits perfectly on a 960x720 screen, however when I move to a larger screen then the content is scaled while the background image remains at its old size, i.e. effectively not acting as a background image anymore.
It would be nice to have an option (e.g. "Scale", "Fit", "Adjust") that will insert the image in its original size and then scale it on larger screens the same way the content is scaled (so background image and content always stay aligned).