I would like to report a player with chat screenshots, how can i upload a picture?

You don't do it here, you should report it in the post-game lobby. Riot uses those reports to track their behavior and punish them appropriately; as long as you report them there, the Instant Feedback System will review their in-game chat immediately. If you feel you absolutely must report something beyond that, submit screenshots in a Support Ticket under the request type I have a question and the category Game & Features::General Questions.