The Musée Historique in Mulhouse has a portrait of Gaspard Weiss, his wife Marie, and their eldest child, Charlotte. It is a pastel, and the family has always believed it to be by Sir Joshua Reynolds, the celebrated English portrait painter. I've never been sure of this - the portrait is unsigned, and it is done in pastels, which Reynolds did not usually use.
I decided to do a bit of research to see if I could come up with an informed opinion on the likelihood of it being by Sir Joshua Reynolds, though I'm by no means an art historian!
The portrait is below. I have no access to the full artwork in colour, so I'm showing the available partial colour one (from the CD of Weiss's music), plus a black and white image of the full portrait.
According to information on the back of the frame the portrait is thought to have been painted in England in about 1777. Charlotte, the daughter in the photo, was born in June 1776 - the child in the artwork would definitely be around 1 or 2 years old (though how they managed to get her to sit still for a portrait is beyond me!) which fits well with that date.
Would Gaspard Weiss have moved in circles which might have allowed him to sit for a portrait with such a well known artist? In a word, yes. Weiss had patrons, students and acquaintances who would have had contact with Reynolds. Through Lord Abingdon, who was one of his students, he became acquainted with Lord Cholmondeley (who also became a student) and Lord Wentworth. Weiss dedicated flute compositions to all three of these men. Reynolds had painted Lord Cholmondeley as well as portraits of members of both Lord Cholmondeley's and Lord Wenthworth's families. Additionally, Angelika Kauffmann, who had bewitched Weiss in his younger years, became very good friends with Sir Joshua Reynolds upon her arrival in London. It is possible that any of these people could have made an introduction for Weiss with Sir Joshua Reynolds.
Did Reynolds actually use pastels? Jeffares in his Dictionary of Pastellists before 1800 (Online edition) observes that Reynolds did not seem to like using pastels, but then goes on to give details of his pastel works, though many of them appear to have been studies for oil artworks. The only image of a Reynolds pastel that I can find online is Head and Bust of a Woman (see below), held by the National Gallery of Art, Washington D.C., which in comparison is a very simple work, though it happens to be done in similar colours to the portrait of the Weiss family. Comparing the two, it certainly isn't inconceivable that the same artist might have done both - there is something about the way the eyes are worked that is quite similar.
To compare other artworks of Sir Joshua Reynolds completed around the same time, there are a lot more options to consider.National Museum Cardiff, is an oil, completed around 1778. It shows Lady Charlotte reclining with a book, while three of her children play at her feet. Lady Charlotte and Marie Weiss share a similar bored, unseeing gaze (to my mind anyway), and even their profiles appear similar. There is drapery hanging from the lefthand side in both paintings also. The Wallace Collection, is an oil completed around 1772-3. While the overall tone of the painting is a lot darker than the Weiss family portrait, the lighting on the actual subject is similar - how the shadows fall etc. Henry E Huntington Art Gallery. Despite the completely different background for the two paintings, the colour palettes are similar, as is the lighting.
So I've picked out some of Reynolds' paintings which definitely show some similarities, but it wouldn't be a proper assessment if I didn't compare the Weiss family portrait with the work of some of Reynold's contemporaries. I chosen the major portraitists of the day, including Thomas Gainsborough, Sir Thomas Lawrence and George Romney. George Romney wasn't working in pastels during the period the Weiss family portrait was painted, so we can count him out straight away. Thomas Gainsborough did use pastels, but most of his pastel works have not survived as he apparently didn't fix them.
So, although it is not an entirely comprehensive study of English portrait painters in the 1800s, I am quite content to consider that the pastel portrait of Gaspard Weiss, his wife Marie, and their infant daughter Charlotte was actually done by Sir Joshua Reynolds. If you are more of an expert on these things than me I'd love to hear your opinion!