Added the below images and caption to the Trombone History Timeline (17th century, 2nd half) and to the Alto Trombone History Timeline.

1658—Görlitz, Germany: Georg Ball und seine Stadtpfeifer, a painting by Johann Geisius, includes a trombone player sitting around a table with a group of musicians. The trombone is relatively small and could be an alto trombone (see detail and full image below; public domain). Special thanks to Tassos Dimitriadis.