UPDATE: The artist behind..

I received Mr. Daniels reply on my email (see this post), asking for permission to use the poster on this website along with some general questions about the poster, this is what I got to know..
Alan Daniels came over from UK to work with Blade Runner, He had up to worked with science fiction illustrations for an agency called Young artists. Most of the work He did for the Blade Runner movie was in the matting process. He spent a lot of time on the set and in the effects studio. Since he had worked on matts before and they liked what Daniels was doing he also got the honour to create a movie poster. He got inspiration for the poster from the set and also from some of the effects used in the movie, This was before Warner Bros. was involved in the Blade Runner movie, things where more controlled by Ridley Scott. Things went on and as Warner Bros. took more control over the movie and they wanted a poster with more of Harrison Ford and not a poster that captured some of the feeling from the movie.
Sadly, Mr. Daniels says that his original art was missing for a couple of years but He has now recovered the poster from someone associated with the Blade Runner movie.
To see more artwork from Alan Daniels, then visit Daniels and Daniels Illustration or soon to be opened website Heavenly Hussys.com

Source: Email.