What are the restrictions placed by copyright laws on reuse of figures in dissertation defense?
If you aren't planning to publish you slides, I would advise you to not worry about copyright law in this case. Copyright law applies to your use of material in slides just like it does any other medium, but there is more possible latitude for fair use claims. That "possible latitude" means legal grey area and lots of possible time and money that can be sunk into a likely fruitless attempt to achieve clarity, since many edge cases may have no official legal determination yet.

Thus, my real advice is this: worry about communicating, not about copyright. Nobody at your defense will report you for copyright infringements. More care is appropriate when posting online, but it is still the case that a) probably nobody will care and b) if somebody does care, the expected action will be to ask you to remove the image from the posted slide.