Knowing whether you can legally use an image or not can be tricky. One problem is that there is not a clear cut rule for determining copyright. There are a lot of factors, including:
Date of the original publication: Current copyright last from the moment of creation to 70 years after the creator's death. However, works created between 1964 and 1977 have a copyright of 95 years from date of publication. Pre-1964 works were originally copyrighted for 28 years, but could be renewed for an additional 28 years. However, even with renewal, most items before 1955 are no longer protected under copyright.
image is going to be use: In most cases, you can use
for educational purposes under Fair Use (see more below) as long as you
give credit to the original owner. You can also use copyrighted works
for criticism, commentary, news, parody and archival purposes. In some
cases, you can use images under a Creative Commons license (see below).
Who created the image: Some works, such as most works created by the federal government, are automatically in the public domain. This means you can use the image (again, make sure you give credit). Also, the rules for "works for hire" are different than normal copyright. If an artist creates an image for a company, it is the company, not the artist, that owns the copyright.
First U.S. copyright, 1790
Fair use allows copyrighted works to be used without permission in instances of "public interest" -- that is, in research, teaching, criticism, commentary, satire, news reporting and library archiving. Now, this does not mean that you can use an entire work and claim fair use. There are guidelines to follow:
How the work is being used -- Are you using the work for commercial or educational purposes? Are you turning the assignment in to just the teacher or are you distributing the work to the whole class? It should be obvious that using an image for commercial use or to distribute does not count as fair use.
Nature of the work -- Ideas and facts are public domain. In some cases, public interest and social usefulness will trump copyright.
Amount of the work used -- Are you quoting a passage in an article or using the whole article? Are you using a still from a film or the whole film? While there may be cases where you can safely use an entire piece, the less you use, the more likely it will be considered fair use.
Effect on the original -- Does your use effect the value or demand for the original piece? You can't use a piece in any way that will negatively effect the original work or copyright owner.
As you can see, even with the above guidelines, fair use isn't clear. In most cases, you should be able to use copyrighted images in your assignments as long as you are not publishing or distributing the image, claiming the image as your own, selling the image, or altering the image in a way that lessens the market for the original image.
That said, make sure to credit the original creator and, when possible, contact them to get permission. Neither I nor the library can be responsible if you infringe on copyright.
Creative Commons is an alternative to traditional copyright. Instead of having so many limitations, Creative Commons actually promotes the sharing, reusing and remixing of works while providing the creator with some basic rights. You can use images under Creative Commons license for practically anything as long as you don't try to pass the work off as your own or sell the image without alterations. Several free image sites such as morgueFile and Stock.xchng make use of Creative Commons.