Yes, it was, like nearly all his paintings.
Yes, thickly is indeed an adverb.Some example sentences are:She lives in a thickly populated area.The thickly-clothed man makes his way through the ruins.
the plains are thickly populated
Thickly snouted pipefish was created in 1814.
I love hot toast, thickly spread with best butter.
large and thickly populated settlements means
Rivers are thickly populated becuse it is generally used for cultivation of crops.
Grass
comparative
wavy
assam
It means to spread or apply thickly as you would jelly on toast.
Real texture physically exists on the art object- consider rust on a metal statue, or thickly-applied paint on an oil painting. Visual texture is the aesthetic representation of real texture- consider a photograph of rust. It shows the texture, but the photograph itself is smooth. Likewise, a smooth painting depicting rust.