I swear I saw an interview with imgur's Alan Schaaf about how it made money, or broke even. So far I've only found this 10/2010 interview in which he only says "it's been really hard to make a profit"

http://thenextweb.com/us/2010/02/10/alan-schaaf-interview-im...

In the 10/2010 interview, Schaff said imgur served 471 million images totalling about 65 terabytes. It had just started pro accounts at that point.