I think most are just posted from the web somewhere. I wouldn't use them unless you ask the poster.
This past month I've been on a Shepard Fairey kick. I'll let you guys google him for yourselves.
He is known for creating propaganda style prints, stickers, and posters that are simply amazing.
A few of them are taken from popular media and reworked (plagarism yes..I know) into something unexpected.
The reason they inspire me is the fact that it takes a different spin on things we've seen before.
My background is a pic of a wall that has been plastered with a rather large Fairey print, if you'd like to see one w/o googling
I have a minor issue with your wording there. You say he "reworked" the pictures. I say he "recreated" the pictures, which in my opinion, is perfectly fine. I see no problem with what he did.