The campaign has been created by Cake and British Airways to welcome home Team GB and ParalympicsGB supporters.
The 53m by 75m image is bigger than 15 tennis courts and 193 London buses and will be visible throughout the build up to the Games.
It also includes the Twitter hashtag #HomeAdvantage to generate buzz and capture excitement.
More than 600 litres of red, white and blue weatherproof paint was used to create the image, which took two days to complete in Thornbury Playing Fields in Hounslow, about three miles from Heathrow.
British Airways estimates it will fly 50,000 ex-pats back to the UK this summer.
The airline has also launched a ‘Flying Britain Home’ campaign offering a quarter of a million pounds worth of flights to Brits that live abroad to fly back for the Games.
A further 50 flights are available to Team GB and ParalympicsGB athletes to bring home their friends and family for an extra boost during the Games.
Ennis said: "I’m thrilled to be the first face that people will see arriving into the UK for the London 2012 Games. I hope it brings excitement to homecoming Brits, and reminds international athletes that we’re ready and prepared! We have the most dedicated fans in the world, who will get behind us every step of the way to give us a home advantage."
Luisa Fernandez, British Airways sponsorship manager, added: "Whether you’re a Brit coming home, or an athlete arriving in for the London 2012 Games, this celebratory message reminds us all of the power of the home crowd and how important it is to get behind Team GB and ParalympicsGB."
During the Games, the airline will host ‘Park Live presented by British Airways’ at the Olympic Park for fans to come together and get behind our athletes.
Dubbed the ‘Hoy Hill’ of the Games, it will offer a place for up to 10,000 fans to watch footage from inside the Olympic venues and see interviews with athletes on the BA Stage.
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