The 53m by 75m picture has been made by British Airways out of biodegradable paint and will be visible for the next five weeks.
The giant image of the heptathlete has been painted on Thornbury Playing Fields.
"I'm thrilled to be the first face that people will see arriving into the UK for the London 2012 Games," Ennis said.
"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."