Feature Request from @Dalai Felinto (dfelinto).
We could also remove the outline completely (when the object is not selected?).
I guess the border is mainly useful when the image is largely transparent.
For the records this aims at mimicking the old 2.7 behaviour where the viewport background images had no outlines.
This conflicts a bit with the idea of allowing background images to only get selected by clicking on their border: D3863#87925.
I agree the border gets in the way when you just want to use it as a reference image though.