Aids Explained In Pictures Copyright For The Sake Of Teaching, Where Does That Fall Under Fair Use?

Copyright for the sake of teaching, where does that fall under Fair Use? - aids explained in pictures

Is it fair to use images from random web sites to include in a PowerPoint presentation for some students? Or is that illegal? Since all non-property diagrams I need are fairly old, and I do not benefit from that, I'm used as a teaching tool. You can access the material itself, to explain only the time.