It is not uncommon for educators to misunderstand fair use guidelines, especially when it comes to publishing things on the web. The only times when we can be 100% sure that something is fair use is when either we own the material, it is in the public domain, or we have direct permission from the creator for a specific use. In addition, media with creative commons license is fair use as long as you follow the guidelines for the type of creative commons license. Usually this is as simple as giving credit to the owner of the work.
Media that does not fall into one of these categories is trickier. We have to weigh the purpose of the use, the nature of the work, the extent of the use, and the possible effect on the copyright holder. Here are links to a couple of documents that help me when I am making decisions about copyright and fair use. I hope they are helpful to you as well.
Copyright and Fair Use Guidelines for Teachers (from the US Copyright office)
Fair Use Checklist (from the Copyright office at Columbia University)
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