I just added the top image and caption (c. 1545) to the 16th century timeline. Giorgio Vasari is a well-known Italian painter, architect, and historian. His book, Lives of the Most Excellent Painters, Sculptors, and Architects, is one of the most frequently-cited art history texts of all time. Vasari springs up multiple times in relation to trombone history. Besides including a couple of somewhat awkward-looking trombones in his art works (see c. 1545 and 1566, below), Vasari also specifically mentions trombone and cornetto in connection with a ceremony for the installation of the garrison of a fortress in Florence (see 1535, below).
c. 1545—Italy: Giorgio Vasari paints a fresco of a quintet of wind musicians, 4 cornetts and a trombone, playing from a window or balcony. The player’s grip of the trombone is somewhat unorthodox and the player is also holding the instrument left-handed. The bell is partially obscured by the player’s head (see below image; public domain) (source: wikimedia commons; Stewart Gardner Museum).
1535—Florence, Italy: According to Giorgio Vasari, Florentine artist and historian, at the ceremony for the installation of the garrison of a fortress in Florence, “The very earth seemed pleased with the Gloria that I heard intoned by the Most Reverend Bishop, who was answered by a multitude of trombones, cornetti, and voices, so that one inclined one’s head owing to the sweetness as when one grows sleepy around the fire. At the conclusion of the oration, the Veni, Sancte Spiritus was begun by harmonies of trombones…” (Cummings 148-49).
1566—Giorgio Vasari’s image of a muse holding an instrument features what Edmund Bowles labels an alto trombone. If it is indeed a trombone, it is a somewhat awkwardly-rendered depiction. The image is associated with a performance of The Genealogy of the Gods for the wedding festivities of Prince Francesco de’ Medici and Johanna of Austria (see below image from festival book; public domain) (Bowles, Musical Ensembles 55).