A Colorado city is inviting dog owners to join together for the second annual "There's No Such Thing as the Poop Fairy" photo contest.

The City of Boulder's Open Space and Mountain Parks Department have invited dogs and their owners to have their pictures snapped to promote the city's new dog waste composting programme.

A spokesperson said the photographers will provide backdrops and costumes, but owners are also free to dress their doggies in any outfits they choose.

The dog voted as the favourite will receive a $50 gift certificate to a local pet supply store and a framed picture of their dog' in costume.

The event is aimed at encouraging dog owners to deposit their pet's waste in provided composting bins instead of leaving the droppings on the ground.