Odd to pick out that portrait as the one thing to say about him after all the things he did in his 45 years as minister to Florence. :)
The portrait can be seen here.
https://libsvcs-1.its.yale.edu/hwcorrespondence/page.asp?vol=24&seq=561&type=bWalpole wrote to Mann, unimpressed with most of the people in the portrait:
"I went this morning to Zoffanii's, to see his picture or portrait of the Tribune at Florence... The first thing I looked for, was you -- and I could not find you. At last I said, 'Pray, who is that Knight of the Bath?' -- 'Sir Horace Mann' -- 'Impossible!' said I -- My dear Sir, how you have left me in the lurch! -- you are grown fat, jolly, young -- while I am become the skeleton of Methusalem!
"The idea I always though an absurd one. It is rendered more so by being crowded with a flock of travelling boys, and one does not know nor care whom. You and Sir John Dick, as Envoy and Consul, are very proper. The grand ducal family would have been so too. Most of the rest are as impertinent as the names of churchwardens stuck up in parishes, whenever a country church is repaired and whitewashed....
"I do allow Earl Cowper a place in the Tribune: an English Earl, who has never seen his earldom, and takes root and bears fruit at Florence, and is proud of his pinchbeck principality in a third country, is as great a curiosity as any in the Tuscan collection."
Mann, also not impressed with the other people portrayed, replied:
"I am glad that you have seen the Zoffany and his portrait of the Tribune. So then it is not true that he was hanged for bigamy, as was reported among the Italians in spite of all I could say to convince them that with us, though he has two wives, it is not a hanging matter. Your opinion of his laborious performance in all the parts you mention, agrees with that of the best judges here, but they found great fault in the perspective, which they say is all wrong. I know that he was sensible of it himself, and tried to get assistance to correct it, but it was found impossible, and he carried it away as it was.... I told him often of the impropriety of sticking so many figures in it, and pointed out to him the Great Duke and Duchess, one or two of their children, if he thought variety more picturesque, and Lord Cowper. He told me that the King had expressly ordered mine to be there, which I did not believe, but did not object to it, but he made the same merit with all the young travellers then at Florence, some of whom he afterwards rubbed out, as old Felton Hervey and one of the Queen's chaplains...
"I should think, too, the naked Venus which is the principal figure will not please her Majesty so much as it did the young men to whom it was showed. As to the question you make me of my own personage, I can only say that everybody thought it like me, but I suppose he took pains to lessen my pot-belly and the clumsiness of my figure, and to make me stand in a posture which I never kept to but then. I remember, for it was several years ago, that I was sadly tired when I was tortured by him to appear before their Majesties in my best shape and looks."