As far as our copious Tumblr research has uncovered, the meme was popularized by user Science Oven (and warning, blog is extremely not safe for work), but then picked up by more users as it rolled along.
The meme seems to have started with Chris Evans as Captain America:
...Continued with The Falcon and Iron Man:
Arguably was perfected with Thor:
And even includes Hawkeye, sporting the sparkliest arrows you'll ever see:
Finally, over the weekend, Agent Coulson was posted after a direct challenge:
For bonus points, users also made The Winter Kitty:
So there you go! Day made. What do you want to see them make next? And are you as confused as we are that there's no Loki yet, because it's Tumblr and the site is 95% Loki GIFs?
Oh wait, here you go: