Online matte paint reference image library MattePaint has shared 1800 free sky images at 2000 x 1000px resolution. All images span a 120 degree field of view, come in JPEG format, and can be used on commercial projects. 

The free images are derived from MattePaint’s commercial library of 360 degree HDRIs, available through paid subscriptions which cost between $19 and $49 a month. You can download the free images here.