Mantra has this feature "Background Image as Matte" https://www.sidefx.com/docs/houdini/props/mantra#vm_backgroundmatte [www.sidefx.com]

can karma do the same? how?

Thanks in advance!

I am looking for this too, do you have any answer?

with karma, you can use Foreground Image in camera or editcamera node.look at the camera help. if you need z-composite, add depth aov for the image."depth" not "deep",and rename the aov name as "Pz". and i found the filter zmax is better.