Pixabay - public domain images
Pixabay.com is a collaborative repository for quality public domain images. People can upload photographs or graphics to the website, and they will be screened to see if they are suitable for distribution. The images so uploaded will go under Creative Commons Deed CC0 into the public domain.
Although it is not possible to claim ownership of any image in its original state or license them granting exclusive rights as they are, it is possible to adapt and use the images also for commercial purposes without attributing the original author or source. For details about this, look the page of terms and conditions.