Here are a few sources of free high-definition photographs that are free to use, both for personal and commercial projects, and without the need for attribution.
Pixabay
Pixabay is one of the largest sources of free photos, illustrations and videos. There are over a million images available for download.
Gratisography
Quirky selection of photography by Ryan McGuire.
LibreShot
Large collection of free to use high resolution photographs by Martin Vorel.