When people say "free" stock photos, there are different levels of free. So let me explain before I give you a list of some "free stock image" sites.

Public Domain

There are a few sites that have collections of public domain photos.  "Public Domain" means that they do not have a copyright and are not owned by anyone.  They are public.  You can generally use them for personal and commercial use without permission.

Royalty-free does not mean Free to Obtain

There are many other photos sites that are labeled "royalty-free". One such example is istockphoto.com. Those "royalty-free" images are not free for you to obtain. These are what I call "commercial stock image sites". In essence, you need to purchase the right to use the image. The photographer still owns the copyright of the image. When you see "royalty-free" it only means that you don't have to pay the photographer per each use of the photo. You still have to pay in order to download it and in order to have a license to use it.

Distinction between Personal and Commercial Use

Depending on the stock photo site, some allow for use only in the personal setting and is labeled "not for commercial use". "Personal use" is very limited. It basically means that you can download the photo enjoy it for yourself. Like maybe it is picture of a sunset, and you like to have a copy on your desk to look at every day.

If you are using photos for your blog and blog has ads that generate income, then it is considered "commercial use". Hence, if you are looking for "free" photos for your blog or article, look for the ones that are free for both personal and commercial use.

May require attribution

Some free stock image site, may require a back-link or attribution next to your use of that photo. In other cases, you have to explicitly give credit to the photo by putting text that says "Photo by (source of photo)" near the photograph. Many images on Flickr is labeled with the attribution license and requires attribution.

May Not Have Model Release Authorization

Another problem with free stock image sites is when you use picture of people where their face and identity is discernible. United States regulations say that if you display such photo in public media or website, you need a "model release" signed from the person in the photo agreeing to have his/her picture displayed publicly. Many free stock image sites not do collect such authorizations from the model. So I try to avoid using photography of people from these sites. Inanimate objects, graphical renderings, and "stick figure" illustrations of people are okay.

The terms of use for NASAImages.org explains it best by saying "Since neither NASA nor the Archive requires a written model release for each image that has identifiable people in it, we cannot guarantee that you will be able to use such images for any purpose you like. Therefore, if a NASA image obtained from NASAIMAGES.ORG includes an identifiable person, use of the image for commercial or other purposes may infringe that person's right of privacy or publicity, and permission should be obtained from the person depicted in the image."

FreeRangeStock does it correctly, because it reviews all is submitted photographs before making it become available. And it requires the the photographer have a model release form signed by any real person in the photos. Furthermore, image can not contain copyrighted corporate logos.

No Re-Sell nor re-distribute Rights

Free does not mean that you can re-sell nor re-distribute. That means you can not put images on mugs, etc and sell the mugs. You can not use images in a website template and sell the templates. You can not grab all the free images and put it on another stock image site (re-distribute). If you need re-sell rights, you have to purchase the appropriate license from commercial stock image sites.

Photographer Owns the Copyright

"Free" does not mean that you can claim ownership. The copyright belongs to the photographer unless he/she has explicitly given away that right. That is also why you simply can not do a web search to pick out some images and use on your blog or website. That is against U.S. copyright laws. By default, the image is copyrighted by the photographer without the photographer needing to do anything except shoot the picture. Simply by pressing the button on the camera, the photographer has been granted copyright of the image shot. In addition, they do not need to place any copyright statement nor symbol in order to own a copyright on it.

Okay, now that we understand what I mean by "free", here are some free stock image sites where you can look for possible images for your blog, articles, and websites. Note that in addition to "free" photos, some of the sites listed may provide "premium" photos (which you have to pay for) placed above the "free" photos hoping that you would purchase those instead. I don't blame them, as premium photos makes for a better business model than free photos.

• sxc.hu - Stock.XCHNG with nearly 3 billion downloads claims to the be leading free stock photo site. Although it has the ".hu" domain suffix which means it was originally a Hungarian website; it is now owned by Getty Images. When you do searches it will list premium images first. You need to scroll down to see the free images that are available. Furthermore, even for the free photos, you have to look to at the licenses and conditions here and make sure what condition the photographer had placed on the image. There are some photographer that stipulates that you notify him or her if you use any of his/her photos or require a credit link. However, many do not. SXC do not have model releases on file, so be careful when using photos of people obtained here.
• Commons.WikiMedia.org - repository of public domain and freely-licensed media content such as images, sound clips, and videos. You can copy, use, and modify any file as long as the source and author is credited.
  
• MorgueFile.com - According to the morgueFile Free License, you can remix and/or use for commercial without attribution.
• FreeRangeStock.com - Can be used for commercial or non-commercial. The site shares Google ad revenue with its photographers.
• U.S. Government Images - most of the image found on these government sites are in public domain. Although the images from NASAImages.org require credit attribution.
• Photos8.com - Also contains public domain photos.
  
• Flickr -- each photographer determines their own rights on Flickr. Some photographers marked their photos as all rights reserved, which means that you can not use their photos. But many release them under the Creative Commons license with the "BY" license, which means that you can use the photo as long as you give credit to the photographer. Other license have the "non-commercial" clause in it.
  
• DesignPacks.com - packs of images from a common theme.
• Public-Domain-Photos.com - As the name of the site implies, all images there are in public domain. But it gives the following warning: "But if some photo contains logos and products you need to be careful. Using someone else's trademark commercially can get you sued."
• PixelPerfectDigital.com - Premium images are promoted in front of the free images. See terms.
• PhotoRack.net - their footer contains a term of use that you can use for commercial projects without backlink.
• PublicDomainPictures.net - public domain photos that are uploaded by users.
  
• Unprofound.com - read their FAQ to see that you can use for commercial purposes without credit display. The funny one says "Yes, you are free use these photos in your company's bus stop advertisements."
• ImageAfter.com - has a lot of texture graphics.
  
• FreeMediaGoo - without a search feature, you have to browse through the photos in the relevant categories.
• PDPhoto.org - The site says "Unless something is clearly marked as being copyrighted, you can assume it is free to use. But if you intend to use an image you find here for commercial use, please be aware that standards for such use are higher. Specifically, you should assume no model release was obtained. And pictures featuring products or property should be used with care."