Final answer:
Heinrich Wölfflin is credited with the art concept of side-by-side image comparison and the formulation of art analysis principles known as the Wölfflin Principles, which help to critically assess artistic qualities in paintings.
Step-by-step explanation:
The art writer credited with introducing the concept of juxtaposing two images for comparison and formulating the principles used for defining and analyzing paintings is Heinrich Wölfflin. His approach, known as the Wölfflin Principles, provides a systematic framework for examining the formal aspects of artworks. These principles are critical when assessing paintings for their complexity, attention-to-detail, and overall artistic qualities, as seen in discussions comparing different artworks and styles.
For example, critics have used such principles to debate the artistic value of Friedrich's paintings, which often feature expansive scenes and attention to detail, such as mountains bisecting the paintings against lighter skies. This method of analysis was also applied in critiques during the early 20th century, notably with Pablo Picasso's reinvention of pictorial space through Cubism.
Lastly, the inverted paintings of Baselitz in the 1970s highlight how the application of color and brushwork can change the viewer's perception of form and space, separate from the painting's content.