William Gilpin was an 18th century priest who defined the picturesque in terms of how landscape paintings should be presented. In his book Essay on Prints, published in 1768, he presented his ideas of the picturesque, that in particular included his ideas on texture and composition. In his estimation, nature itself was not able to create the perfect composition and often needed a helping hand from the artist by adding an extra tree or some other object to "perfect" the scene.
By contrast, Wordsworth, in "An Evening Walk," clearly celebrates nature as it is in all of its raw beauty. Wordsworth therefore debunks Gilpin's ideas of the picturesque through finding even the presentation of nature in the deepening darkness a thing of beauty that cannot be "improved" by man's intervention. Note, for example, how he presents this picturesque scene:
The latest lingerer of the forest train,
The lone black fir, forsakes the faded plain;
Last evening sight, the cottage smoke, no more,
Lost in the thickened darkness, glimmers hoar;
And, towering from the sullen dark-brown mere,
Like a black wall, the mountain-steeps appear.
The picturesque for Wordsworth is not something that can be viewed as needing man's intervention. Nature is what it is and it is insulting to suggest that it needs "improvement" in order to make it truly picturesque. Wordsworth in this poem pays homage to the powerful and majestic elements of nature that refuse to be tamed or domesticated by man. His use of conventional, picturesque scenes from the Lake District as well as scenes that were not considered to be picturesque in the same way confirms his views.