Here are some links about copyright and fair use I came across this morning. They contain good information and "lots of opinion". Helps to define fair use, derivative works, transformation, etc. After reading them, it appears we should give credit to the originator even though our work may be very transformative. For example, it we take someone's photograph, convert it to a line drawing, use that as a tracing pattern to tool a piece of leather, we should give credit to the originator, even though there is really no copyright infringement due to the transformative nature of our work. It is good etiquette. Further, the originator should be asked for their permission, again as good etiquette.
http://blogs.scientificamerican.com/compound-eye/infringement-or-fair-use-have-a-look/
http://blogs.scientificamerican.com/compound-eye/infringement-or-fair-use-part-ii-the-opinioning/
http://bizarrocomics.com/2013/07/21/fly-by-drawing/
Tom