These days, anyone can 'take a selfie' on their phone and get an instant image. In the 18th century, having your 'picture taken' would mean getting an artist to paint or sketch the image. Only the wealthy could afford to pay a professional artist for the work required. Alternatively, a talented amateur might sketch or paint his friends and acquaintances. Some portraits of musicians are engravings, often found in contemporary editions of their works. In these, there can be no case of mistaken identity. With paintings, sketches and other artworks, the identity of the sitter can be doubtful.
There are three known portraits which are claimed to be Antonio Lotti (or have been claimed). There is also an image on the internet of Benedetto Marcello, which keeps being used erroneously as an image of Lotti.
The Conservatorio di Musica San Pietro Majella in Naples contains a gallery of portraits of musicians through the ages. There are two portraits worth considering. The first one is said to be a portrait of Antonio Lotti, though it is a 19th-century portrait, and the artist is unknown. Working after Lotti's death, the unknown artist would have had to have copied some original image, which has otherwise been lost (and not copied elsewhere). The clothing perhaps seems later than early 18th century.
(Currently, this is the only digital image of the painting!)
The second image in the Galleria of the Conservatorio is currently said to be Johann J. Fux, though it bears no relation to any other portrait of him, of which there are a number. The image is sometimes claimed to be of Lotti, most recently in an article on Lotti by Prof. Stefano Lazzoni in Storia della Musica, vol. 2 (Jaca Book, Milan 1995).
Again, this image was painted by a 19th-century artist, Ernesto Papa, so it is unclear on what he based his portrait. The clothing seems also a little modern.
The musician and artist Lorenzo Giovanni Somis (1686 - 1763) made a great number of chalk sketches of musicians with whom he worked. Somis was based in Turin for a considerable period.
This is inscribed in pencil on the back with the words 'Antonius Lotti'. The problems with this attribution are that the pencil was not really a contemporary implement, so likely to have been written later. Also, there is no record of Lotti ever working in Turin, and this is part of a bundle of sketches made of musicians at the Turin court of Carlo Emanuele III, King of Sardinia.
On the plus side, Somis may have travelled to Venice at some point to study with Vivaldi (though the Somis family produced several violinists, so documentary evidence can be confusing.)
This portrait, then, is the most credible, while still maintaining some doubt.
Please get in touch if you know where the original drawings are.
Si prega di mettersi in contatto se si sa dove sono il ritratto originale.