Using photos obtained from academic homepages in a research seminar talk

To avoid all questions, and in the absence of a clear license, ask the person for permission to use a picture. They might even have a better one that they would prefer be used.

For a deceased person, ask the university, either through their former department head or a university publicity office. If they can't grant you a right to use it, they can possibly put you in contact with someone who holds the rights, such as their estate, for example.

I don't guess anyone would turn you down, and this is simple enough to do.

In the United States, and perhaps elsewhere, copyright exists as soon as a "creative work" is "fixed in a tangible medium." So, in the United States every modern photograph is covered by copyright. The Creative Commons license is just that, a license to use the work. If there's no mention of a license, then the assumption should be that use is not allowed without prior approval.

Copyright law in the United States, and perhaps elsewhere, includes the doctrine of fair use, which in some cases permits unlicensed use of material under copyright. The trouble with that is, fair use claims are adjudicated on a case-by-case basis, so one could be sued and even prevail, but have to bear the cost of defending the suit. The U.S. Copyright Office has a lot more information on fair use.

Your best course, as Buffy has already written, is to contact the person whose image you want to use, and ask.

I guess I should add that images of people who are deceased are a different and messier can of worms. The decedent's estate likely owns the image. Images of, e.g. a famous actor, might be owned by a corporation. I haven't worried about images of people who died before 1920 because there were major changes in U.S. copyright laws that made 1920 an important date, but this isn't legal advice, and I could possibly come to regret not worrying.