Prince William and Kate Middleton released the black-and-white photo on the prince's 11th birthday on Monday.
The Duchess of York was originally scheduled to be in Australia in October.
Buckingham Palace said last week that the royal was hospitalised for minor injuries and a concussion following an incident at her country home.
"I am sure you are watching over us always," Ferguson said of the late Princess of Wales.
Two people were arrested a day before the summer solstice, which typically draws many visitors to the site.
The new photo comes just one day after the Princess of Wales made her first public appearance in six months.
The young royal couldn't help but get funky as he listened to bagpipe music at Trooping the Colour in London.
The Princess of Wales attended Trooping the Colour on Saturday, and made a very special appearance alongside her three children.