Of course, if a work is in the public domain, copyright is no longer valid and making a case for fair use isn't necessary. But the rules around what goes into determining what is in the public domain can be just as confusing as what constitutes fair use. A work can enter the public domain through the following circumstances: expiration of copyright, failure to renew copyright, deliberate placement by the creator in the public domain, and copyright laws that prevent the work from being copyrightable at all. To begin to understand why this is so tricky, take a look at this chart from Cornell University detailing the public domain stipulations in over 50 different scenarios.
If you're consulting an image collection, you can expect to see a copyright statement indicating whether the work is in the public domain--just make sure that information is coming from a trustworthy source! In particular, check to see whether the collection includes a standardized statement like one from RightsStatement.org, which exists so that users like you will know without a doubt whether an image is protected under copyright or not.
While copyright is one possible limitation on appropriating images, there are many other reasons why using an image or other information might not be the right choice. This can be the case even if you have a compelling fair use argument, the image is in the public domain, or you're considering distributing an image that you legally have the rights to.
When deciding whether your use is ethical, consider some of the following issues. Even if you're not certain of the answer to some of these questions, weigh the repercussions of your use and consider contacting someone who might have more information.
1. Consent: Do the subjects of the image appear to have consented to their likeness/an event having been photographed? Would the use of such their image only serve to perpetuate this harm? This can fall partially under Right of Publicity laws, which vary state by state. And remember, just because the law permits the use of an image doesn't mean that use is inherently ethical.
2. Privacy: Does the image in some way violate someone's privacy? This differs slightly from Right of Publicity, which can protect the privacy of the subject's likeness or name.
3. Community accountability: Was the image intended for use and/or viewing for people within a certain community? Would sharing or using that image be harmful to members of that community? Example: Zine Librarians Code of Ethics.
4. Cultural sensitivity: Are there cultural protocols outside of copyright governing the use/viewing of certain images? Remember that images circulating outside of their cultural contexts may not have any indication of these protocols. Example: Traditional Knowledge Labels.