Sorry if this is old news to people but I've just come across the rather wonderful short guide to Rock Art on the Northumberland Council web site.
This is a free PDF, very easy to read, lots of great photos and a great introduction to Rock Art in general, link below
http://www.northumberland.gov.uk/idoc.ashx?docid=fd2d871c-4b55-40c4-818b-142ab9c5d246&version=-1