Leonar-Werke Arndt & Löwengard
This logo identifies the photographic paper.
Leonar was the brand name for cameras and photographic material made by Leonar-Werke Arndt & Löwengard in Wandsbek near Hamburg, Germany. The company ran from 1907 (and earlier under another name) until 1964 when it merged into Agfa.
Postcards with the Leonar logo (which has a few variations) comprise what look like both commercial and private photos.