List of websites that offers free images. This list is compiled mainly on work by PapiChowClapz from Reddit : what are the best sites to get images for your blog.

Image license types & concepts :

CC0 : Creative Commons "No Rights Reserved" : images with this license belongs to the public domain and are free to use (both personal & commercial).
CC3 : Creative Commons "Attribution" : images are free to use (both personal & commercial), you can build upon the images and you can change the images. However you MUST give appropriate credit.
Royalty free : you pay a flat fee for a resource only once and can then use the resource in multiple ways, many times and forever.
Public Domain : content that isn't protected by copyright law and therefore free to use without obtaining permission. Images can come to belong to the public domain in 3 different ways :
- The image is assigned tot he public domain through a cc0 license (or similar).
- The image is not copyrightable.
- The copyright for the image has expired.