Is it okay to upload slides based on others papers?
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.