I'm confused now... if I receive a magazine in the post and like a hair style in it, and take the magazine to the barber/hairdresser/hairstylist, and ask them to eg: shave my hair similar to like in the photo, is that illegal?  If I tear out the sheet, and do the same, is it illegal? If I cut out only the photo or only the haircut is that illegal? [As of proof reading this, I've realise that copying the hairstyle might be break copyright and IP law too.  Just to play it safe, when I next shoot a model, I shall have the make-up artist sign a makeup-release  ]

( Re lending a physical book to a friend to read and then have it returned, or not, or if I sell it on, I am not aware of this being illegal, or requiring licensing.  This does not mean I'm right or wrong, so I'm asking on this? ... However, just as when we used to play football at school, and some kid brought his ball, we did not pay a rent/licence/etc to play with that ball.  He owned the ball, and we did what we wanted with it...  I draw the line at, not being able to 'reproduce' the content of the book, and then sell it.  ie: The book that I have purchased, maybe be sold on, however I'm not entitled to photocopy it, or re-write it for that matter.  However in the case of school-projects, Thesis's, etc, proper referencing/bibliographing needs to be done. )  [ If you really want to be anal about things, no original content is really original.  Where do you draw the line, on what is original content in a book, such as "how to use your camera".  Lets assume an author was taught at school/college/other about aperture and shutter and then focuses (no pun intended) on ISO in the book.  Self taught on ISO, but to describe ISO, makes reference to shutter/aperture.  Some authors credit their teachers, but not all do.  I'm not saying that its right or not too, but its practically impossible, since we learn daily thought human-interactions. .. enough of that off topic RANT ]

Back to my first example, magazine are disturbed, and the companies would be glad of us, the general public 'showing' their adverts around.  I'm hoping my hair-dressing example is not illegal, because the next scenario is a bride, tearing out sheets from various magazines, 'these are the flowers I want for my wedding', 'this is the dress style I'm going for', etc... is that illegal?

So far what I've seen of Pinterest is that, as some others have mentioned, is its a public billboard.  If you tear out a photo from a magazine, hang it on a wall in you bedroom/house, is that illegal?  What about when your friend comes over and sees that 'famous singer/actor/other' hanging on the wall.  Is that act now illegal?  Pinterest seems to link to the hosting website, and does not hold the content itself from what I've seen.  Please correct me if I'm wrong!?  Is google illegal, or google-images illegal?  It points to the hosting-site.  Pinterest can be seen as a 'google' where users pick what makes it to the page and not.  When it comes to facebook/etc. and someone downloading a photo off another site, and then placing that in their album, then that, I believe is illegal.  HOWEVER sharing/posting a site with the content I would like my friends to see, is something else, and I don't believe that is illegal.

Its a very fine line, between illegal and legal use, and it seems that people draw that line at different points.

Sites like www.tineye.com do not check photos pixel by pixel, rather they use algorithms which will still find 'copied' works.  These works could have been cropped, enhanced, modified or used as as part of another image.  ( I believe the google-image-search tool works in the same way ).

If am not mistaken tineye offer services and other tools specifically for the 'Agencies' needs, and if am not mistaken too they also encourage portfolios/galleries/albums/depositories (call them what you want) to be added to their (ie: tineye's) image-banks.

It might not be the Agencies job to check duplicate/copyright works, however it is in their interest... unless their interests are in making profit from the money from held-back-sales.

TinEye http://www.tineye.com/  might be able to help with tracking down some images, including derivative/mashup works.