The world of free stock photos can be a difficult one to navigate. If you're using a paid image repository like iStockphoto or Shutterstock, your rights and responsibilities are clear: You pay for the rights to use the image, and no attribution is required.

However, if you're choosing to go the free route--and really, why wouldn't you?--it can be confusing to know how you're allowed to use the photos, and what (if any) attribution is required.

This post will give you a list of 20 sites that offer 100 percent free images for use on your blog or social media posts. And the best part? All the sites below offer photos under the Public Domain or the Creative Commons Zero (CC0) license. This meansno attribution (link) is required.

In case you're unfamiliar with copyright categories, here's a brief overview of the three main types:

Public Domain--Images in the public domain are works whose copyrights have expired or whose creators have relinquished all rights to the works. These are free for personal or commercial use. There is only one exception to the "free for all use" policy for public domain photos: If you're using an image of a recognizable person, you may need to get his or her permission to use the photo. Some sites offering such photos will already have obtained a model release, so if in doubt, consult the license agreement that comes with the image.

Royalty Free--When images are offered royalty-free, this simply means that the purchaser pays a fee and can then use the image without paying additional royalties or licensing fees. This also means the purchaser doesn't have to give attribution. This is the model used by paid stock photo sites.

Creative Commons--Photographers who license their images under Creative Commons (CC) can choose how their images will be used by specifying a particular CC license. For instance, a CC0 license means the owner has relinquished all rights to his or her works, while CC 2.0 requires the user to provide attribution.

CC0 is the most flexible license by far: "CC0 enables scientists, educators, artists and other creators and owners of copyright or database-protected content to waive those interests in their works and thereby place them as completely as possible in the public domain, so that others may freely build upon, enhance and reuse the works for any purposes without restriction under copyright or database law."

Please note that the sites listed below are all either public domain or creative commons, and none require attribution. I've done my best to specify which license each site uses. However, please do your due diligence before using images from the sites below, particularly if plan to use them for commercial purposes.

Creative Commons 0 (CC0) and Public Domain image sites.

1. Pixabay--A huge database of public domain images

2. New Old Stock--Vintage photos from the public archives

3. Unsplash--10 new high-quality photos released every 10 days. Released under the CC0 license.

4. Foodie's Feed--High-res food images. Free to use without attribution; however, the may not be resold.

5. Death to the Stock Photo--Free images for commercial use. Delivered monthly to your inbox. You do not have the right to claim these photos as your own.

6. Magdeleine--One free high-res photo every day (and access to a full repository of images). Use the handy filter to find images that are either public domain or require attribution.

7. Public Domain Archive--All images are completely free for personal or commercial use, no link or attribution required. I personally use this for my latest startup Due for some of the amazing whitepaper images that we're putting together for the big launch.

8. Good Free Photos - Public domain images taken by the owner of the site. Offers some good location-based images.

9. Free Range Stock--Free high-res images, registration required. It is suggested you link back to the site and give credit to the photographer, but it isn't required. You cannot resell, distribute, or claim ownership of the images.

10. Pickup Image--Searchable database of public domain images.

11. Photogen--Free for personal or commercial use, but not suitable for resale or redistribution.

12. Gratisography--Free collection of amazing images taken by photographer Ryan McGuire. All photos provided under CC0.

13. Skitterphoto--License-free photos, free to use under CC0.

14. Life of Pix--High-quality, public domain images with no copyright restrictions.

15. Pexels--Searchable database of CC0 images.

16. Morgue File--A huge repository of free photos. You're free to use the images for personal or commercial purposes, but you cannot claim ownership of them.

17. SplitShire--Free photos with no copyright restrictions.

18. 1 Million Free Pictures--Free amateur public domain images. What they sometimes lack in quality they make up for in quantity.

19. pdpics--Public domain images taken by their in-house team of photographers.

20. Flickr: Creative Commons--Access to all Flickr photos sorted by license. This allows you to find images under the public domain, non-commercial license, attribution license, etc. Make sure you select 'Public Domain' to find images that don't require attribution.

Half the battle in finding great images for your blog or social media posts is in knowing how you're allowed to use them. Can I edit them? Can I use them without linking to the original source? Hopefully the list above makes the whole process a little bit easier for you.