The Duke of Cambridge waas also ful of praise for the portrait.
Prince William said:: "It's beautiful, it's absolutely beautiful."
The Cambridges later attended a private breakfast reception at the London gallery to mark the unveiling.
More top news
A man's been given an 18 week suspended prison sentence after his three dogs died in a hot car in Peterborough.
Pippa Middleton took civil court action against a "person or persons unknown" after pictures were allegedly taken from her iCloud account.
A school in Essex has stopped giving homework to pupils because teachers don't have enough time to mark and prepare lessons.