Fair use:

From the US Copyright Office

"One of the rights accorded to the owner of copyright is the right to reproduce or to authorize others to reproduce the work in copies or phonorecords. This right is subject to certain limitations found in sections 107 through 118 of the copyright law (title 17, U. S. Code). One of the more important limitations is the doctrine of “fair use.” The doctrine of fair use has developed through a substantial number of court decisions over the years and has been codified in section 107 of the copyright law.

Section 107 contains a list of the various purposes for which the reproduction of a particular work may be considered fair, such as criticism, comment, news reporting, teaching, scholarship, and research. Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair:

1. The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes
2. The nature of the copyrighted work
3. The amount and substantiality of the portion used in relation to the copyrighted work as a whole
4. The effect of the use upon the potential market for, or value of, the copyrighted work

The distinction between fair use and infringement may be unclear and not easily defined. There is no specific number of words, lines, or notes that may safely be taken without permission.

Acknowledging the source of the copyrighted material does not substitute for obtaining permission."

Use of images by individuals for educational purposes is generally considered fair use. If money changes hands or the image is used for publication (of some sort) then image users need to receive approval from image creators or owners before use to avoid copyright law violation.

Carefully read the verbiage at the origin of the image. Often image users rights and responsibilities are clearly defined there, as are the means for asking permision for use of desired images. Look for the terms "copyright" or "usage".