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Copyright law is a minefield that defies clear definition. The following is an attempt to clarify it. This is not legal advice.
Copyright provides five exclusive rights for the creator of an original expression--a creation that has a modicum of creativity and is in a tangible medium:
The right to
copy the work
distribute the work
create derivative works (e.g. translations, adaptations, abridgements, etc.)
publicly perform the work
publicly display the work
Any use of the creation by someone other than the copyright holder that falls in any of these five criteria infringes copyright.
Copyright is granted automatically: one does not have to apply for / register / or have a copyright notice. One secures copyright merely by creating something that has a modicum of creativity and is in a tangible medium.
Not copyrighted It can be freely copied, used, and distributed.:
Any work whose copyright term has expired is in the Public Domain and therefore not protected by copyright.
USA: works created before the present year minus 95 years (i.e. before 1925 in 2020)
US Government works are not copyrighted.
Works from countries that do not have a copyright agreement with the US are not copyrighted.
If the creator explicitly abandons a work's copyright, then that work is not copyrighted.
A post-1925 (as of 2020) image of an artwork that is in the public domain (i.e. the artwork was created before 1923, but its photographic reproduction was created after 1923) is copyrighted (unless the image is merely a slavish / artless reproduction without creativity).
If you want to use copyrighted images in research, you need to get permission. To do so, you need to find and ask the copyright holder (usually the creator). Finding the holder, however, is often difficult, as such information is rarely in the public record and the rights may be divided among multiple parties.
To find the original copyright holder, you can search the following:
Reverse Image Search: if you have a digital image file or link but don't know where on the World Wide Web it originally came from, you can do a RIS on your image file to find its original web location
images.google.com: click the camera icon and either enter the URL or upload the image file
tineye.com: either enter the URL or upload the image file