According to CBR’s coverage of the J.G. Jones Spotlight panel at Comic-Con, the artist used himself as the reference for the iconic Flash cover on Final Crisis #2:

Finally, who did Jones use for photo reference for the Flash image on the cover of “Final Crisis” #2? “I used me!” But self-reliance has its price, Jones revealed, because the posing process slows things down. “I have to pose, take a shot, look at it, pose, take a shot, look at it…” Fortunately for readers, Jones’s patience tends to pay off.