There appear to be no records of a professional artist who worked under the name of J. Lebreton (20th Century); he or she may have been a part-time painter, who had some other type of week-day job or profession.
Possibly Lebreton is a descendent of Louis Lebreton, French (1818-66); a naval surgeon who produced watercolour sketches from his voyages, showing 25 of them at the 1841 Salon; he then sold them as illustrations in several periodicals. He also produced a body of c. 350 lithographs.
Or a relative of Constant Le Breton (b. 1895), artist and wood-engraver, who illustrated numerous books, including volumes of poetry by Baudelaire and Gerard de Nerval and works by Oscar Wilde and Voltaire. He exhibited at the Salon, and painted the portraits of Pétain and Derain.