In response for a request on C18-L in 1997 for a summary of the articles on illustrations of Richardson's novels, I provided the following, and place it here because according to the plates of an edition of Clarissa the frontispiece to the first edition as edited by Angus Ross for Penguin, the well-known painting by Joseph Highmore, it is of "James Harlowe accusing Clarissa after her return from Miss Howe's. I would like to comment that although the picture has been placed at this turning point in the story, there is no such scene in the novel. Highmore's painting corresponds to the scene before Clarissa left for her visit, one in which there was a general conclave, and in which the brother played a prominent role by protesting just enough to cover Clarissa's going with suspicion, but not enough to keep her at home (Ross Penguin, p 57-8).
For descriptions of the actual pictures, reproductions and hard information of all sorts I'd begin with T. C. Duncan Eaves's "Graphic Illustrations of the Novels of Samuel Richardson, 1740-1811," HLQ 14 (1950-1), 349-83; also his "The Harlowe Family" by Joseph Highmore,' A Note on the Illustration of Richardson's Clarissa. HLQ, 1 (1943-4), 89-96. Also interesting is Marcia Allentuck's "Narration and Illustration: Problems in Richardson's Pamela, PQ 51 (1972), 874ff. The picture by Highmore of Pamela telling nursery tales to her children has garnered much commentary because of the moral paradigm it contains (on which see Toni Bowers' _Politics of Motherhood_), but there are other interesting approaches to illustrations. Ellis Waterhouse in his Painting in Britainand Sacheverell Sitwell in his Conversation Pieces situate the pictures stylistically and place them in the larger more famous and prestigious traditions (of painting--there was never as much prestige in making a picture for a book). Before she began her annual production of melancholy novels about suicidal women, Anita Brookner wrote a marvelous book on Greuze in which she discusses the same kind of themes one finds in the illustrations of Richardson's novels. There are a slew of articles over the years in Eighteenth-Century Studies on the theme of absorption in French painting which comes over into the English. This relates to the theatre, for the pictures are not so much realistic as figures on a stage; here you could look at Diderot's writings--and Aaron Hill. Finally another kind of article I found interesting was one on the illustration of French novels, Paul L. Grigaut, "Marmontel's 'Shepherdess of the Alps' in 18th Century Art,' Art Quarterly 10 (1949), 31-47. There is a strong erotic current as well as class and nostalgic basis in these illustrations that's worth looking into.