Fascinating detail! Also, that's actually a beautiful painting, and strikingly different from official portraits Pesne did of the royal family. This is a painting I really like.
did she come across it during a visit?
Not unless Algarotti gave it to Fritz before 1744, when Ulrike got married and moved to Sweden. At this point, Sanssouci didn't exist yet, but Fritz could of course have hung the picture elsewhere. The 1771 visit to Brandenburg was Ulrike's first after her marriage, so she can't have seen the painting in Sanssouci at the point when Fritz is making this will. If she knows it, she must have seen it before 1744 in Potsdam and Berlin. Now, since Pesne lived in Brandenburg since F1 hired him as court painter - and was among the people FW didn't fire (he just cut the salary Pesne was receiving by half) -, this wouldn't be that difficult. Ulrike was portrayed by Pesne as were her siblings, after all. (There is a portrait of her hanging in Rheinsberg today.) Maybe, just maybe, instead of Pesne coming to her rooms, she was allowed to visit his studio and pose there, which means she could have seen the painting there?
Alas, though, dates argue against it. Pesne was appointed court painter by F1 in 1711; this painting hails from 1706. At which point young Pesne was living in....Venice. Where Algarotti is from. Which would explain why Algarotti owns a painting of the Prussian court painter to give to Fritz that the royal family doesn't own already. (Meaning young Pesne probably sold the painting back then to some Venetian noble and Algarotti, who could have seen it in Palazzo X, aquired it there.
As to why this painting of a peasant girl with cleavage, well, as my Aged Parent noted, there is no lack of female half nudes or nudes at Sanssouci anyway, enough to make him question Fritz' sexuality again. And it is a beautiful painting. I think it's probably no more complicated than Algarotti knowing Fritz would like such a good Pesne.
As to why Fritz should think Ulrike would like it, well, see above. The only way she could have known it was if she'd seen it before 1744, which could be the case if Pesne didn't sell it while in Venice but kept it and brought it with him to Berlin. Which is also possible, though in that case I question why Algarotti owned it before Fritz did.
Pesne painting
did she come across it during a visit?
Not unless Algarotti gave it to Fritz before 1744, when Ulrike got married and moved to Sweden. At this point, Sanssouci didn't exist yet, but Fritz could of course have hung the picture elsewhere. The 1771 visit to Brandenburg was Ulrike's first after her marriage, so she can't have seen the painting in Sanssouci at the point when Fritz is making this will. If she knows it, she must have seen it before 1744 in Potsdam and Berlin. Now, since Pesne lived in Brandenburg since F1 hired him as court painter - and was among the people FW didn't fire (he just cut the salary Pesne was receiving by half) -, this wouldn't be that difficult. Ulrike was portrayed by Pesne as were her siblings, after all. (There is a portrait of her hanging in Rheinsberg today.) Maybe, just maybe, instead of Pesne coming to her rooms, she was allowed to visit his studio and pose there, which means she could have seen the painting there?
Alas, though, dates argue against it. Pesne was appointed court painter by F1 in 1711; this painting hails from 1706. At which point young Pesne was living in....Venice. Where Algarotti is from. Which would explain why Algarotti owns a painting of the Prussian court painter to give to Fritz that the royal family doesn't own already. (Meaning young Pesne probably sold the painting back then to some Venetian noble and Algarotti, who could have seen it in Palazzo X, aquired it there.
As to why this painting of a peasant girl with cleavage, well, as my Aged Parent noted, there is no lack of female half nudes or nudes at Sanssouci anyway, enough to make him question Fritz' sexuality again. And it is a beautiful painting. I think it's probably no more complicated than Algarotti knowing Fritz would like such a good Pesne.
As to why Fritz should think Ulrike would like it, well, see above. The only way she could have known it was if she'd seen it before 1744, which could be the case if Pesne didn't sell it while in Venice but kept it and brought it with him to Berlin. Which is also possible, though in that case I question why Algarotti owned it before Fritz did.