A photo in an article with a caption that reads:
Law enforcement officers during a raid of a gambling house in the 1940s. At upper left is Mike Elliott, whose campaign for Multnomah County sheriff would be financed by gambling racketeers. (Oregonian archive)
Why is it "at upper left"? I thought it is idiomatic to use the preposition on and with the definite article, "on the upper left". Is at upper left correct?