To flex on people for no apparent reason just because his self esteem is extraordinary low. He also may he tendencies to swear, be violent, and to step on other peoples shoes just because he is a massive dick, but that doesn't mean he HAS a massive dick.
To pull a Riley is to do something super cool like accidentally making a pattern on the flash game "Circle The Cat".
You can also pull a Riley by making really really funny puns or by just being super rad.
To interrupt an ongoing, text-based, conversation by sending a screenshot of little to no connection to the topic at hand, thereby disrupting the flow and leaving everyone confused.
A: Did you read the chat from yesterday evening? It was hilarious, everyone was having a blast!
B: Yeah dude, it was great. Until someone sent that pic of their Clash of Clans base, why did they have to pull a Riley like that?