[xep-support] scaling a background image to completely fill a block

[Dave]

We have an interesting situation which requires us to use a background 
image of unknown dimensions.
We'd like it to scale uniformly such that its entire block is covered. 
So rather than having some of the background left blank we'd like parts 
of image to be clipped away.

In Essence, we are trying to achieve what css background-image-size: 
cover; does.

Any help would be appreciated.

-dave