more about 'being inspired by' others' art

I asked the photographer Matt Wright (see his website for his most famous photo of Obama in cowboy hat), if I could use the hat in my collage of Barack and Hillary on a Tandem. He said ok!

copyright issues regarding Obama Hope poster

Here's an interesting piece from lawyers at Stanford about the right of an artist versus another artist: a photographer, regarding the use of a photo as a base for a poster. A poster that became very famous! What do you think? Was the photographer ripped off? Is the image different enough for the artist to claim there is no copyright infringement? Personally I ask the photographers for permission to use their work.