The giant image of Ennis, the size of 15 tennis courts, includes the words "welcome to our turf" and aims to spark a Twitter buzz via a #homeadvantage hashtag.
Cake created the concept as part of activity to draw attention to the role of British Airways as the official airline partner of London 2012.
The campaign is also aimed at welcoming home Team GB and ParalympicsGB supporters as the airline estimates 50,000 ex-pats will return to the UK this summer.
Luisa Fernandez, British Airways sponsorship manager, said: "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."
British Airways claims to have supported over 1,500 UK Sport athletes and their families from 28 Olympic and 20 Paralympic sports with flights to train and compete abroad.
During the Games the airline will host 'Park Live presented by British Airways' at the Olympic Park, which is being positioned as a place for fans to go before or after watching an event or as a destination in itself, in a similar manner to Henman Hill at Wimbledon.
The parkland area can accommodate up to 10,000 visitors and will feature giant screens showing the live action, as well as a British Airways stage for interviews with athletes.
Follow Matthew Chapman at @mattchapmanUK