The concept of the "picturesque" was created by the English clergyman, artist, and writer William Gilpin (1724 - 1804) in his 1768 art treatise Essay on Prints, in which he defined the picturesque — rather tautologically — as "that kind of beauty which is agreeable in a picture."
In later publications Gilpin developed the concept more fully. The picturesque may be thought of as halfway between the beautiful, with its emphasis on smoothness, regularity, and order; and the sublime, which is all about vastness, magnitude, and intimations of power; the picturesque must combine aspects of both of those. A picturesque landscape would have characteristics of roughness (which includes textured or variegated surfaces) — indeed, Gilpin wrote that "roughness forms the most essential point of difference between the beautiful and picturesque" — and an absence of regular or linear elements, and would effectively orchestrate a number of additional compositional elements: distance, light/shadow, "variety," and perspective. In Gilpin's words, "Picturesque composition consists in uniting in one whole a variety of parts...."
Of considerable importance in the idea of the picturesque is the idea that it is an aesthetic of effect; for all intents and purposes, it is an aesthetic that almost does not exist independently in nature, but only in its perception by the viewer -- and particularly in its arrangement. This is why Gilpin's definition of "picturesque" is tautological; it is not so much a naturally occurring phenomenon as it is a created one, created primarily by painters but also by trained observers. (Gilpin in fact wrote an essay on "Picturesque Travel," effectively explaining how to "create" the picturesque in your mind's eye when you view landscapes.) As Gilpin wrote in "On Picturesque Beauty," beautiful objects are "those which please the eye in their natural state" while picturesque sights "please from some quality capable of being illustrated in painting."
The ease with one category of aesthetic blends into another is evidenced by the fact that Gilpin himself often referred not to "the picturesque," but to "picturesque beauty."
"... we picturesque people are a little misunderstood..." — Wm. Gilpin
Claude Lorrain, Pastoral Landscape (1645)
Claude Lorrain, The Herdsman
William Gilpin, View from the Bank of a River
William Gilpin, Ruins at Canterbury, from Observations on the coasts of Hampshire, Sussex and Kent, relative chiefly to Picturesq Beauty (1774)
The picturesqueness of ruins, which exhibit the "roughness" Gilpin found so important, is made manifest in Gilpin's explanation of why a conventionally beautiful object, such as a formal piece of architecture, is out of place in a picture: "A piece of Palladian architecture may be elegant in the last degree. The proportion of its parts -- the propriety of its ornaments -- and the symmetry of the whole may be highly pleasing. But if we introduce it in a picture, it immediately becomes a formal object, and ceases to please. Should we wish to give it picturesque beauty, we must use the mallet instead of the chisel; we must beat down one half of it, deface the other, and throw the mutilated members around in heaps. In short, from a smooth building we must turn it into a rough ruin. No painter, who had the choice of the two objects, would hesitate a moment."
Thomas Gainsborough, River Landscape
Richard Wilson, Pastoral Scene with Musicians by a Classical Ruin
The Claude Glass
Picturesque Painting [The Sister Arts]