Le Dejeuner sur l'herbe [The Luncheon on the Grass 1863] is a painting by Edouard Manet [1832-1883]. It is a familiar painting, now recognized as among the most important pre-Modernist paintings.
The history of painting over the last 500 years is a record of a succession of styles, a development both of technique and of subject matter. The gradual emancipation of the artist from the limitations of formal strictures, as well as of the range of acceptable narrative, during the 19th Century, is an account of challenge and defiance of cultural norms, with each stage setting a higher bar of permission, for those who would follow. The idea of revolution in art, is born in the late 18th and early 19th Centuries, and to a large extent, we're still carrying on this struggle of resistance and renewal through invention and discovery.
How must the public have felt, in 1863, upon first seeing this large [about 7x12 feet!] canvas at the Salon des Refuses? Clearly intended to shock, the picture is also a commentary upon classical subject matter, updating and placing it in ironic contrast to its putative historical models.
Le Dejeuner sur l'herbe of Manet
Its immediate impression is of a sort of metaphorical tromp l'oeil, of a classical nude placed inside a contemporary setting, without any clear connections to context--rather as if a Renaissance nude from a mythic vignette had been plopped down in the middle of a Paris suburb. Nudity or semi-nudity in classical painting had long been a proper subject of painting and sculpture, but nearly always in the context of a remote reference, either from classical myth, Biblical scenes, or legend. This familiar distancing of the profane from its public allowed artists to explore raw human form without straying into obscenity or vulgarity.
Manet's painting clearly intends that we should see this scene in just that way, as a sin against good taste and as a reaction to its classical models. The stark contrast of the cool, pale-skinned nude with the dark tones of the sylvan background, and the clothing of the two male figures, the harsh lighting and staged positioning of the limbs--all suggest defiance, and prurient disregard. There's clearly something pornographic about the presentation, as a challenge to propriety as well as to the canons of doctrinaire taste.
Comparing Manet's version to the later one by Claude Monet--which seems tame and impressionistically calm by comparison--will give some sense of the huge contrast of intention.
Le Dejeuner sur l'herbe of Monet 
Though the figures are drawn in a realistic manner, they seem stiff and posed, as if in imitation of some formal, theatrical posture. This rigidity would be normative in a lot of classical scenes, but here it seems completely out of place. We have difficulty inventing a narrative that would explain the nude's disrobing, or her presence between the two conversing men. Is her nudity a violation of her sex, or is she a projection of the aesthetic preoccupation of the figures, who seem to be stand-ins for the painter himself. Or is she just a piece of furniture in a satirical cartoon?
It's the incongruousness of the nude's presence which determines our reaction. She clearly doesn't belong in the painting, in the same way that she doesn't belong in a real picnic in the France of the 1880's. It's like a temporal displacement, a forced enjambment of contradictory contexts. This incongruousness has more connection to later artistic movements--Surrealism or Dadaism--than to any discernible tendency of its time. The painting may seem shocking, or humorous, or defiant, but it doesn't strike one as "beautiful" or graceful or cheerful, or even melancholy or moving. It's an aesthetic statement, one intended to draw a line in the sand, either an end or a beginning, depending upon your point of view. In a way, it's more typical of how Warhol or Lichtenstein might conceptualize it, than how any critic or viewer would have in the 19th Century.
Outside the area of the figures, the painting seems pretty sketchy, an afterthought, the brush strokes casual, even careless. This contrast between the hard clarity of the foreground figures against the pictorially drab background also underscores the sense of imposition, of a truncation of the historically separate modes. Manet seems to be emphasizing the disjunction, without making any overt attempt to connect the opposing contexts. This kind of deliberate exaggeration and disjunction has much more in common with later absurdist depictions than anything else of its time.
Unless, of course, we are willing to apply the same Modernist or Post-Modernist criteria to earlier, classical works such as those of Giorgione, whose two pictures here are commonly accepted precursors of the Manet work, both of which display much of the same kind of accepted "techniques"--the skewed perspectives, the incongruous nudes, the staged quality of the narrative, and the sketchy metaphorical landscapes which form the backdrop of the drama. If we think in a relaxed way about these earlier efforts, it's easy to see how naive, absurd and unreal they are. That we should, on the one hand, see these 16th Century canvases as typical masterpieces employing standard mythical subject matter, while viewing the Manet as a shocking challenge, tells us much about how later developments and critical accommodations of the emancipation of art from the clichés of previous dogma have altered how we view works within the progression of historical development.
Were audiences in the 16th Century as offended by the nudity of such canvases as these by Giorgione, as audiences in the 1880's must have been by Manet's? Did they see art in the same way we now tend to do, as natural reflections of the spirit of the time, rather than as evidences of a kind of divine inspiration whose purpose was to inspire them to imagine another kind of (ideal) reality?
The Pastoral Concert of Giorgione
La Tempesta of Giorgone