Riddle's picture books include The Great Escape from City Zoo, The Royal Guest, Nobody Owns the Moon, My Uncle's Donkey and Unforgotten.
The cartoon collections are selected from his work as the weekly cartoonist for Good Weekend magazine (Sydney Morning Herald and Melbourne's Age), a position he held for nearly ten years from 1997–2006.
[1] In 2009 he won the Patricia Wrightson Prize in the New South Wales Premier's Literary Awards with Ursula Dubosarsky for their book The Word Spy.
In 2022, His first solo exhibition, Silent Light, was held at the Blue Mountains Cultural Centre to considerable acclaim and 38 paintings were sold.
In 2023, his Wynne Prize entry was included in the Salon des Refuses, at the SH Ervin Gallery.