Copying images without permission may be a copyright violation. The person who creates an image (or a piece of text or music) owns that work and has the right to decide how and when it is used, including where and when it can be copied and distributed. There are numerous factors must be considered to figure out whether copying and posting of an image is permitted.
If something is in the public domain, anyone can use it freely. HOWEVER very few images on the internet are in the public domain. Usually they are limited to images:
- created before the early 1920s,
- created by or for US government agencies, or
- that the owner (usually the photographer, the artist or their employer) has explicitly dedicated to the public domain.
That is practically none of the images that appear on the internet. Unless something explicitly says it is in the public domain, you have to assume it is not.
Despite repeated claims to the contrary, the placement of images on the web DOES NOT place them in the public domain.
Self Created Images
If the poster made it, she owns it (unless she did it in connection with her employment).
If the owner of an image gives you permission to use it, it is licensed and you can use it in the manner and to the extent that the owner has explicitly allowed. But only in that manner and to that extent. Any other use is a copyright violation.
If you find an image, check the site (e.g., the LEGAL section) to see if there is a license, and what it allows. If there is no indication, that means IT IS NOT LICENSED.
There are some specialized licenses, such as Creative Commons, that allow extensive use, but have certain requirements about how a work is distributed and what notices are required.
This is the hard one. The use of some copyrighted materials may be allowed without the owner's permission if it meets the criteria of the Fair Use Doctrine. This allows certain uses that are not likely to diminish the interests of the owners of the copyright. For example, certain specific educational uses are allowed. Other uses may be allowed if they are not likely to compromise the intent and commercial interests of the owner. There are four factors that are generally considered and weighed to determine if a particular use meets the fair use criteria:
- The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes
- The nature of the copyrighted work
- The amount and substantiality of the portion used in relation to the copyrighted work as a whole
- The effect of the use upon the potential market for, or value of, the copyrighted work
As you can see, these factors are not simple yes/no questions. There is often no bright line to indicate whether a use would be permitted. A fuller discussion of fair use can be found at the Copyright Office website here.
[What follows is merely an viewpoint, and not a description of the policy of this site, or a statement about the law or the reaction that an owner of an image may hold.]
One approach that may meet the fair use criteria is the inclusion of images from commercial sites if you also include links to those sites. For example, if a manufacturer or distributor shows a product image on its site, they are likely to either own or have permission to use that image, and are not likely to object to you copying that image if you link back to their site. This forum does not encourage promotion of any commercial product or site, but such a link, for illustrative purposes only, would probably not raise the ire of this forum or the linked company. It would also be a good idea to insert a notation that makes clear that the link in not an endorsement.
Lifting images from sites that rely on images to promote their own interests is unlikely to meet the fair use test. Lifting copies of images that the owner seeks to sell (e.g., stock photos, plans to be download for a fee) is definitely not allowed. Use of images from sites that offer information similar to this site (competitors) is likely to bring the Wrath of Khan (or the Revenge of the Lannisters, depending on your own pop-cultural star) down upon you.
Your own stuff is the best. Your friends' stuff they said you could use, next best. Everything else, take care and think about the factors laid out in the fair use doctrine. Often, if an unauthorized use is objected to by an owner, removal of the offending image may be enough to resolve the problem. But tread cautiously.