Depending on the scenario, many images you may wish to use in your lectures are permitted under Section 110 of the US Copyright Act (please see http://www.copyright.gov/title17/92chap1.html#110), as long as you present the images directly to your students in either a face-to-face setting or in a controlled-access environment, such as Blackboard. Only students enrolled in your class should be able to access the lecture in which you are using the material from the slides.
