Freddie Mercury singing Radio Ga Ga at Live Aid.
(Quickest image I could find, sorry)
Aha, now I get it; it's short-haired Mercury. (I've always been more familiar with the long-haired version of him from 71-80)
I chose the image because, to me, it was a truly defining rock moment, one of the greatest live acts owning two stadiums and millions of viewers across the globe. Plus, every time I watch Freddie and the 100,000 plus at Wembley clapping in unison during the chorus, I get tears, every time.