Not sure how the person did this, but somehow they did. This should be fixed. Makes chat hard to read. The small icon in chat is fine. Whatever they did to make the image take up the whole chat window is bugged.
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Your picture
Just gotta sanitize input. 
I can blur “you” and change the picture if it’s needed (And the other players at the same time) 
I really dont mind at all. Blizz just needs to sanitize input and not parse it.
Even if they don’t, not many people know about it. Highly doubt anyone would actually abuse it to hide general chat. I just had some fun with it today. You can also always ignore the person doing it. I assume many people ignored me today after i posted it lmao. No powerlevels for them i guess! 
It was just a proposition
Appreciated brother. It’s not like we’re naming and shaming here, since this is actually just a simple bug, and honestly it looks like it’s clientside pictures anyways, but you can do fun stuff like posting chicken in your status on friends list. xD