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 ;D
Yes, they can. Unsplash photos tend to get used a lot though.
Another source is http://www.publicdomainpictures.net