Trombone images from the 18th century are somewhat difficult to find, largely due to the fact that the trombone itself was a bit difficult to find during that era. However, I’ve recently added a pair of interesting images to the Trombone History Timeline (18th century). They’re shown below, along with their captions/references.
1716-1727—Germany: Protestant minister Daniel Pfisterer keeps a personal sketchbook in which he sketches various aspects of his surroundings. Included is a drawing depicting numerous instruments on and around a table. The trombone is on the right (see below image; public domain) (source: Recorder Home Page).
1774-77—Rome, Italy: Giovanni Volpato creates copper engravings of paintings by Giovanni da Udine found on the colonnade of the Loggia of Raphael in the Vatican, originally painted in 1517-1519 (see 16th century timeline). The instruments are grouped in trophies (or decorative clusters), the grouping that features trombone representing a shawm band, a common 16th century ensemble. Although the engravings by Volpato are generally considered good copies, they reflect some 18th-century characteristics, like the round stays on the trombone (see below image; public domain) (Myers, Instrumental Trophies).