This is what I would do in the Mus POV
If its rank and I see this in my pregame chat.
I know this guy will be a problem so I would just get an image like the one you have now.
Dodge and take either the -3 lp or -10lp loss (Better than losing -18 + LP/MMR)
Wait it out
Because if he's toxic in lobby, he will probably be toxic to you ingame. Not worth the headache
What to do now...
Oh wait...
You got a photo
What to do with that sexy photo....
Load up www.na.leagueoflegends.com
Find the support tab.
Close out your extra tabs to avoid lag (Remember Mus perspective)
You create a ticket but first check out all the things riot has to offer and get sidetrack
Sometime 10 minutes later, you work on that ticket.
Not an ordinary ticket that you get at the county fair
A ticket named "Support' and with a nice sexy number
Throw that juicy photo there and explain to the Riot that he/she made you feel bad.
The more you the write, the more they feel for you.
Riot knowing that they love you
Will give you there usual response.
"We cannot discuss punishments to other players but know this, anyone toxic will not be treated differently" or something like that
If something happened or not.
We as players do not know
Since privacy is something we agree to when we load up the TOS for the first time.
But hopefully the report will mean something.
Note: I wanted to do a different approach for response.