(BBC) The photo, showing the child being carried on a woman's back, was taken three weeks ago by a Spanish tourist.
Madeleine disappeared on 3 May from a holiday apartment in Portugal.
Her parents' spokesman, Clarence Mitchell, said Gerry and Kate were aware of the photo and it had been passed to authorities to study.
Spanish websites, including the Spanish radio station COPE's site www.cope.es, have published the picture.
The tourist, Clara Torres, said the similarity to Madeleine was striking.
She said she decided to hand the photo to police on Monday after reading of other recent possible sightings.