One other point with using an image for a sky: the horizon may look wrong.  With the Bryce sky you can adjust have, cloud height, fog, etc.  Using a image, you have to simulate that.

I didn't put much time in this and it shows.  However, with some tweaking it should look decent enough.

Anyway, what I attempted was to position a cube that stretches across the horizon and intersects the 2D plane and the ground (or water) plane.  I then used a volume material to try to get the haze look.

The great thing about Bryce is the many different ways one can get some pretty good effects.