https://creativecommons.org/

Any time you use a picture to illustrate your work, you must use royalty free pictures and always give credit.

Here are some websites where you can find images labelled for reuse:

1. Google Images labelled for reuse. Guide on how to get them.
2. Wikimedia Commons
3. Pixabay
4. FreeImages
5. 4 Free Photos
6. Photobucket
7. Sunipix
8. Pixmac
9. PhotoRack
10. ImageCodr
11. ImageBank
12. PhotoPin