This is a photomanipulation...as you can see :P. for blending i've used blending options set on lighten for blending the sky with the rainbow, erase tool and history brush for merging smoothly the images that illustrate the mountain and the horses. Used the warp transformation tool for distorting the rainbow in order to look much more real. In the end, I've concentrated on the color scheme, added some shadows and lights on the horses using the dodge and burn tool. Mainly, this is the entire creation process. Enjoy!
credits and thanks: