I wanted to ask the same question today, now that I found it has been written before, I think I'll write an answer instead. As a disclaimer, I personally don't like most of the stuff I write below and I am not really giving a *, but thats pretty much how it is.
First of all, StackExchange does not count as an educational site, as it is clearly commercial, remember that there are ads here and a whole company working behind it.
This question has an image, with the copyright notice removed. While I don't think it was intended by the author of the question, this clearly is rude behaviour.
This answer extensively uses copyrighted images.
All images on the Internet have to be considered copyrighted as with their creation the creator gains it. Unless he stated that it really is public domain, an image is to be considered non-free. How do we know if an image is free or non-free if there is no notice? We don't… So we have to assume it is non-free.
There are a lot of answers that contain images, even with the copyright notice included. As long as the author of the question doesn't state that he contacted the source, we have to assume that he is not allowed to.
I don't find the answer, but there was one, that had a lot of the gif'ed images taken from exrx.net. I don't think that they would be happy about this if they knew.
Images are nice, I agree with that. But do some randomly collected Images really make up for the name of the exercise in combination with a link to a site where the exercise can be found? (I am aware of link rot)
I have no final answer to this, but I would prefer if we removed images from answers, if they are non-free. I know how stupid this might sound, but on the other hand, we make StackExchange responsible for legal issues if we do nothing.