An 18th-Century Representation of the ʾUmayyād Mosque

The credit for the oldest representation of the ʾUmayyād Mosque goes to the Russian traveler Vasilief Gregorovitch-Barsky, who toured the Near East between 1728 and 1747 C.E. Older paintings of Damascus, including the mosque, do exist, notably the famous "Reception of a Venetian Delegation in Damascus" in 1511, but Barsky's was the first independent illustration of the temple. 

Essentially, Barsky got the salient features of the edifice right: the location of the minarets, the transept surmounted by the Dome of the Eagle, the Treasury in the courtyard, and Ǧayrūn's Gate with the stairs leading to the fountain "an-Nūfarā." Two factors might explain the error of replacing the crescents above the minarets and the central dome with crosses:

1. Most orientalists and travelers prior to the 19th century were under the impression that Caliph al-Walīd had simply converted St. John the Baptist Church to a mosque with no substantial change in its layout and essential features. Such was the view of the French diplomat Chevalier Laurent d'Arvieux (1635-1702), who visited the Levant in the late 1650s when he opined that "this mosque is constructed after the manner of our churches; the Turks have changed nothing in it."

2. Christians and Jews (and all non-Muslims) had been denied entry to the mosque until the mid-19th century. With that in mind, Mr. Barsky's representation was as accurate as one could get at the time. It goes without saying that the mosque was amply described by numerous Muslim authors in the past, most precisely by the Andalusian Ibn Jubayr in the 12th century, but his and his peers' were merely written narratives devoid of illustrations.

René Dussaud. Le temple de Jupiter Damascénien et ses transformations aux époques chrétienne et musulmane. Syria. Archéologie, Art et histoire. Année 1922  (3-3)  pp. 219-250. 

Gérard Degeorge. Syrie. Art, Histoire, Architecture. Hermann, éditeurs des sciences et des arts 1983.