https://unsplash.com/
Free (do whatever you want) high-resolution photos.
For 10 new photos every 10 days.

I can't find any USE info

Can these be used for "commercial purposes"?

If so, it's another great source 

Yes,  they can.  Unsplash photos tend to get used a lot though. 

Another source is http://www.publicdomainpictures.net