Wikipedia says it's a German postcard dated 1900. But says nothing about the artist.
Does anyone know about the artist behind this work?
The author is unknown if the file's wikimedia commons is anything to go by, but digging a bit deeper using Google image search seems to variously attribute the original to a painter named Lindberg, to H Zabatari/Hans Zatzka, and possibly others.
Be wary that the sources might very well reflect hearsay without checking or, like this answer, googling. That being said, H Zabatari/Hans Zatzka seems like a better bet based on this slightly more credible looking and better researched source (an auction page for an original):
The Guardian (c. 1918) was painted by H. Zabateri as a postcard (Austria?). Original postcards with the artist's name (credit) can be purchased from collectors. Several prints of this painting have been attributed to "Lindberg". Lindberg copied the orginal painting and used brighter colors, detail changes, and simplier figures.