Can I post a photo of my dead friend in social media?
Also, should people tell each other about the death of a friend? Will it be bad for the dead person if I'm sad?
Firstly, sorry for your loss. My following answer will consist of my opinions but I hope it will guide you finding answers to your questions.
As someone who is strictly careful about posting photos on the Internet (because I have seen a lot of bad things done with others' photos), I would never post a photo of someone else on the Internet without their permission. Since you can't ask your friend's permission in this case, better not to post your friend's photo.
If the people you will talk to also know the dead person, why not inform them and talk to them about the death of that person? Talking about death and dead people is one of the ways to remember death and adjust our lives accordingly. (the more we remember death, the better the world will be). But it is also better not to overdo it, as it may run the risk of speaking badly after the dead person.
Your feelings will have no effect on a dead person, your prayers may.