puting a sky image with the Mr Combo sun and sky it is a very pain in the but, but there is some work around, normally I put the sky in post but if you want an animation I rather put a dome sky and make a work around that
Jeff Patton explain here.
If you still want to use the image map in the mrsky you have to pull way up the RBG values of the image, remember that Mr Sun and sky use "a lot" of illumination and with the photographic exposure it is more evident, so your image must have the same "output" values