Hi there,
I worked on this image two days and I hope you like it.
I emitted the balloons with two particle systems and the background is actually a matte paint, as you see in the second image.
The images I used:
http://www.snoron.com/walls/mountain_peak-wide.jpg
http://www.snoron.com/walls/snowy_mountains-wide.jpg
http://www.snoron.com/walls/mountains_view_landscape-wide.jpg