I would never, never use slides for a course.

That said:
I do sometimes show my student pictures taken from the web.
For example, I recently showed this picture to the students in my group theory class in order to illustrate the isomorphism between $A_5$ and the group of symmetries of a dodecahedron.

Also, I sometimes prepare animations with Geogebra that I then show during class. Here's an example (click and drag the blue node). Of course, it's even better to create the graph in front of the students: Geogebra is good for that. My philosophy is that students should be shown things being created, not ready made. But I'll admit that this is not always possible...