Go to your C drive > Programs(x86)> steam > userdata > large number here > 760 > remote > Number > Screenshots

The number depends on the game. Keep clicking until you find the Spiral Knights folder.

Scroll down and find the screenshot.

I think first you need to calm waaaayy down and get your head on straight. Yelling and making a fuss over getting a Warning isn't going to make the GMs take you seriously, in fact- I'm quite sure the result will be the complete opposite.
I'm honestly surprised that you reacted this violently when you didn't even get banned. If the latter had happened then, ok, you've earned a small right to be angry. However, most problems have solutions and even in that situation you could still submit a support ticket to OOO....but if I'm to understand you correctly, that didn't happen.
After reviewing the TOS I was surprised to see that some language is actually accepted (correct me if I'm wrong GMs ;)) in Spiral Knights, but this is supposed to be a mature game so... However, harassment (as Ironskull mentioned) isn't allowed. You could have, in your anger been inadvertently harassing the guy. But that would need the logs. In any case, the guy that reported you about it is probably going to have a huge wake-up-call if he ever sits around Haven 1 for a few minutes.
That said, all that happened was that you got kicked from a group by an idiot and no doubt complained about by that same idiot. What you experienced was probably just an automated warning that's sent to every player that's complained about. whoop-de-doo.

Hmm, well, let me tell you about the game that I came to SK from:

Said game was a 'family' game, open to little kids, teens and adults- with the majority of the population being people with no logic, no sense of sportsmanship, and general stupidity. In fact, my sister, who still plays (and is playing at this very moment) is yelling at someone for cheating in pvp. Or something. She's really 'angry' typing so it can only be that XD Anyway, in this game false reports were commonplace and, well, if you were mature and just wanted to have fun you were forced to accept the whole attitude that the people who kicked you, Tfwacker, have. Just report and switch servers was pretty much the mantra. However, many people didn't like the way the others were acting (or anything else, for that matter, if some paid-for in game money was lost then it would result in threats to leave the game or general ugliness toward the GMs- which is why, when people left this game over the update I rolled my eyes) and as a result went on to the game's community forum (much like this one, but the game's GMs didn't manage it like ours do) and started to rant. And rant. And rant. Many were banned from the forum and even the game simply because of the way they were speaking to the GMs and all of those people lost the respect of all who were mature. Naturally the mature people had complaints too. But we submitted support tickets if there was anything wrong and when we did it we kept levelheaded arguments. We got our problems fixed and they didn't.

Why did I tell you this story? Well, I think one of the things one has to keep in mind when talking to GMs is that forcefulness and frustration=headache and actual levelheaded arguments=progress. The GMs have enough to worry about already so....ah.. yelling at them isn't going to help. I'm not sure exactly, it's just something I've learned about the way support works.