"This painting is IMHO one of the most valuable documents of costume history. The gallantry and coquettery of the 18th century has made it possible that a peek at someone's underclothes was recorderd for future generations. By the way, novels and fake diaries often also divulged details that were considered juicy - the 18th century is probably the best documented era in the underwear department.
The lady to the right presents us with a nice view of the Watteau pleats of her contouche which she wears in the casual style: caught up in loops of cord fixed to the inside of the skirt or pulled through the pocket slits. Below, the relatively narrow low-level petticoat (or decency skirt) is visible. On top of it, she might have put on small paniers.
The left lady gives even more interesting insights: We see that her stocking, probably made of knitted silk, is tied with a ribbon just above the knee. These ribbons were popular trophies among the gentlemen. On top of a white petticoat with a flounce, she wears a blue jupe and obviously even a robe the bodice of which is protected from the makeup by a neglig jacket. Where it opens we can see the front-laced corset. The black dot on the temple is a mouche." (Quoted from the Women's Fashions in the 18th Century)
18th Century Women's Fashions: La toilette by François Boucher, 1742
A Flickr group site with a huge number of 18th century photos of original objects, engravings, paintings and reproductions--see below
If you are interested in a LARGE number of photographs of 18th century fashions from paintings, engravings and replicas modeled by modern re-enactors go to the Flickr group site 18th Century Fashion. Pictures are being constantly added and updated at this site.