In some seminar classes, one student presents a paper selected from top conferences or journals with the help of slides. Usually, there is a lot of discussion about the strengths and weaknesses of the papers. The plots or figures are usually not prepared from scratch for the slides, but are copied from the paper by taking screenshots. But is it legal or even ethical to publish these slides online?
It is almost certainly a copyright violation in the US. It's probably also defensible via a fair use argument (in court!). I think that the odds of getting sued or DMCA takedown noticed are pretty low, but I still wouldn't encourage the practice of uploading the property of others to the Internet without their permission. That's a reasonably big ethics issue for me. The easiest thing to do might be to use a placeholder figure "See Figure 2 page 6", or similar, in the uploaded material. You might also try asking the publisher for reproduction permissions. They all have a process, and it ought to be free and easy for purposes of commentary.
If said slides are available publicly e.g. from the author's homepage, there is no problem (but you must credit properly, which I assume is done anyway if this is part of a seminar class).
If the slides are published e.g. as add-ons to the paper on the conference webpage, and are behind a paywall there, presumably your school has permission to access them, so placing copies (or linking to them, which might be preferable) from a page private to the seminar at the school should be legal (you might need to check the precise conditions). If the page for the seminar is open to the public, it is obviously a no-no.
In any case, I am not a lawyer, and even if I was, I don't have enough details on the specific case to be able to give any advise. Legal advise can only be given by a lawyer retained by you, in full knowledge of the case. Details we as laymen consider irrelevant might make all the difference legally.