The law regarding this is called the Digital Millennium Copyright Act of 1998 ("DMCA") ... which covers the legalities about posting images and other material taken from other sites and magazines ... usually in violation of copyrights. Here's a link to the information given by the U.S. Fed. Gov't. and a copy of the act.
http://www.copyright.gov/legislation/dmca.pdf The act itself starts on page 12 ... not sure this is it, in it's entirety. But this is the final authority on the issued of copyright violations via electronic media.
It really takes an attorney to decipher this ...
And here's an intersting artilcle about the act which might shed some light on the issue.
http://blog.wired.com/27bstroke6/2008/10/ten-years-later.html
I am not an attorney ... so don't accept my advice as the best advice or even accurate, because I'm just passing on information that I have been told by other people. Contact your attorney, to protect yourself.
But I will say this: I know the law does not talk about crediting ... because crediting does not change the fact that images were taken without permission. But crediting is customary, to hopefully encourage copyright holders (usually the photographer, the magazine or the advertisor ... a blogger or the person who scans something is not the owner of the copyrights) to accept that the images are posted without specific permission. It helps a lot, it seems. Make sure you identify the copyright holder(photographer, magazine, etc.) in addition to the site where you took the image. And you certainly should have a policy posted somewhere stating that copyright holders can contact you and you will remove the infringed content immediately. That helps too.
But nothing beats getting written permission to post it first ... that is really the only time you are within the law, I would say.
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