What is the least amount of effort needed to ensure no copyright issue of using online pictures in lecture notes/slides I have noticed that in many university's course notes/slides, particularly those in the CS department, and in particular those dealing with computer animation/graphics, a large amount of instructors tend to reuse images from textbooks or the media (Hollywood) without credit or from other courses. There is an immense amount of examples, just pull up any course notes nowadays to see.
This seems to be a wide spread and common practice according to Can I use images showing up in Google search for my presentation slides without violating any copyright?
Here is my question:

What is the least amount of effort needed to ensure no copyright
issue of using pictures in lecture notes/slides?

Obviously these instructors think it is ok to say something in class, or perhaps the image/picture is so famous that it is immediately obvious where it is from.
Does it suffice? If not, how to properly credit a picture of something that just exists?
For example, I want to use a scene from a 3 hour long movie, say lord of the rings. This picture does not appear anywhere on the internet except in the movie itself, which I paid for. What is even the citation format for an image that appears in 1 micro-second of this movie? It seems to be an overkill to perform a full citation to the movie, is it not?
As another example, I want to use @offense_name_handle's original content on DeviantArt/Tumblr. Do I say, &quot;This image belongs to @offensive_name_handle&quot;? Obviously I do not want to post this in my slide. But I want to use the image nonetheless. How to deal with this grey area?
Google has some suggestions, but I really don't want to contact a lawyer for a duck image. https://support.google.com/legal/answer/3463239?hl=en
In any case, what is the least amount of effort needed to provide a citation that avoids copyright issue?
Or should I always try to generate my own content or use published academic reference to be safe?
Use images that have a CC0 (public domain) license. That way, you can use them for any purposes even without having to attribute them to anyone.
For instance, use https://search.creativecommons.org/ and select the license &quot;CC0&quot;:

Or https://free-images.com/ seems to be a search engine specifically designed for CC0 images.