Ask HN: Where can I find images without copyright for my scientific talk?

In the course of my computer science program at university I have to give a talk every now and then. I like my slides to be full of pictures and not walls of text. Are there good sites with stock photos that I can use for free in my talks? These slides will not be posted online so I doubt it matters much anyway, but somehow I would still like to not use pictures in my thesis talk to which I don't have a permission.