I suppose the nicest shop in Brighton selling pictures has to be Art Republic on Bond Street. They sell prints and posters of everything from the last 100 years. From expressionism to surrealism and from pop art to the latest Banksy piece, they are all here.
In the last couple of years they have gone in for the more contemporary/urban/grafitti artists in a big way. I think they must have contacts with Pictures On Walls as they carry most of their artists.