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.