I'd like to share my 40 sky backgrounds little collection.
Format: jpeg
Resolution: ~5000px
Ratio: 16:9
(https://dl.dropboxusercontent.com/u/21099839/hdri-maps/hdrmaps-com-free-ldr-thumb.jpg)
I hope you'll find it usefull in some projects.
Enjoy it!
Download free sky backgrounds (http://hdrmaps.com/freebies/free-ldr-skies-vol-1)