and here is a photo (1931) wrongly, in some sources: attributed to him,
I suspect the confusion lies in the fact Flammarion published a book with the picture, and a computer search turned up his name in proximity to the photo. Below is a good example; Flammarion is listed as the artist for this book sometimes, but a simple glance at the title shows the root of the confusion.
It is amusing that the artists's name on the cover is just Steinlen, when his name is Theophile Steinlen. The editeur though, has his whole name on the cover of this lovely book, published in 1898. Here is another illustration from the pages within:
I don't know how much money Ernest Flammarion made on his brother's books, but another source, Henri Poincare, Impatient Genius, (Ferdinand Verhulst, 2012) says he made "a fortune" on Bibliotheque de Philosophie Scientifique, (under the editorship of Gustav Le Bon ((1841-1931)) whose idea it was). He published in this series these volumes by Henri Poincare:
La Science et LHypothese (1902)
La Valeur de la Science (1905)
Science et Methode (1908)
Dernieres Pensees (1913).
That firm (or the successor which bought it out in 2000) also recently published Choupette, about Karl Lagerfeld's cat, but that is for another post.