It's an issue I deal with often too!
Here's a general thumbnail overview:http://www.collegeart.org/ip/qa7
Here are "5 Famous Infringement Cases
The Associated Press vs. Fairey case startled the heck out of me, personally, though if Fairey had PAINTED his image (that is, changed the photo into another medium: painting, sculpture, etc) he might have been untouchable.
PHOTOSHOP is tricky because one is still manipulating / filtering the original art, even if part of a collage afterwards.
Stay away definitely from recognizable faces! I did a cover for an anthology made up from numerous bits and pieces: somebody owns that image of the Moon but who can say "That's MY image of the Moon" etc. Flames are flames and from three sources. An astronaut torso and a helmet? Skulls? Tentacles? All safe swipes imo. http://photos.modelmayhem.com/photos/121006/17/5070c8f637dbf.jpg
But I knew I had to use an original face or the model with that face (without permission) could tear it all down! So that's a local girl Ashley.
Though i have used faces before where I blended one model's lips to another's eyes and still another's cute lil nose and thought I was safe ... except then you realize how freaking alike some famous model face are! You take THREE DIFFERENT faces and it still looks like the same one supermodel! http://photos.modelmayhem.com/photos/111106/02/4eb6646d35e43.jpg
Hence, ALWAYS keep a record of each resource image and each LAYER, so you can prove "this wasn't any one face, it's different faces, not just her (the girl suing me)" etc. "This one image is TWENTY images" goes a long way towards proving it's a new, unique, original piece of art ... and thus legal fair-use.
That's mah story an' ah'm sticking to et!
Maybe that helps somehow? But it's a tricky vague issue, like you said!