A self-taught artist, Barreto named Russ Manning, Hal Foster and Warren Tufts as his three main artistic influences. When he was 15 years old, his portfolio under his arm, he went to each and every newspaper in Montevideo looking for a job. With a Richard Lionheart biographical comic inspired by Foster's Prince Valiant, one of his favorite comics) as his strongest work and which he had intended to sell outside of Uruguay, he finally found a job at the newspaper "El Día". The editor for the newspaper's children's magazine ("El Día de los Niños") liked Barreto's art, but he asked him to do something more Hispanic. Thus, an adaptation of the Spanish epic poem "Cantar de Mio Cid" ("The Lay of the Cid"), was soon published in the magazine, scripted and drawn by Barreto, aged 16.