Using photos obtained from academic homepages in a research seminar talk I am preparing a research seminar talk in an academic discipline which I'll give online (with Beamer) at University X.
I like to show images of the people whose ideas have gone into my work. Most of the time I can get images which have been clearly released under some form of Creative Commons License, allowing me to use it provided I attribute the copyright holder. However this is not always possible, and sometimes the only photo I can find of Person Y is on their personal homepage at University Z, where there is no mention of copyright on the image, or whether it may be used by others.

Is one allowed to use the photos found on the personal webpages of people working at academic institutions for non-commercial purposes such as in the slides of a research seminar talk, if there is no explicit mention of copyright or limitations on its use by others?
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.