I just added the below image and caption to the Trombone History Timeline (18th century). Special thanks to John Manganaro, whose new site, horniconography.com, contains a wealth of excellent materials.
The majority of 18th century trombone iconography originates from Germany and Austria. This reflects, I believe, a general trend in trombone activity. However, there are exceptions. For more on the trombone in Bologna during this time period, see Guion, David, “The Missing Link: the Trombone in Italy in the 17th and 18th Centuries.” Early Music, Vol. XXXIV, No. 2 (2006).
1742—Bologna, Italy: A miniature from Le Insignia degli Anziani Consoli (vol. XIII, c. 156, 1742) depicts a reception in honor of Carlo Emanuele III of Sardinia in the Palazzo d’Accursio’s Galleria degli Anziani. Among the instruments pictured in the orchestra is a trombone (see detail and full image below; public domain) (Bologna Archivio di Stato; special thanks to John Manganaro, horniconography.com).