If in doubt about whether or not you have permission to use a photo, you likely don't. Unless the rights for an image explicitly state that you can use it how you would like to, you don't have permission.
In Oh snap, can I (legally) use that photo?, Peg Fitzpatrick talks about legal photo usage online.
From +Peg Fitzpatrick
Oh snap, can I (legally) use that photo?
Everyday we search for just the right photo for our post on our blog, Facebook or Google+ from a wide variety of sources. But do you have the legal right to use the photo just because it’s online? No. Can you get in trouble for this? Yes. Let’s look at what you can and can’t do to remain a good citizen of the web.
One of the biggest misconceptions is that just because the photo is online that you can post it anywhere. That is NOT true. Don’t do it.
Go read the article. Peg makes some excellent points.
Thing is, this isn't just about photography. It's about all intellectual property – stories, poetry, articles, blog posts, photographs, drawings and other images, art, and the list goes on.
Even though the attitude, especially amoung the younger generation who grew up with always having Internet, it's not legal to take whatever you want whenever you want just because.
Look to see if rights usages are indicated. All rights reserved means that the owner of the copyright – usually the person who took the photo, created the artwork, wrote that article – doesn't allow anyone to use their stuff without explicit written permission. You may see something like a Creative Commons license, which means that the work might be available for some usages in some contexts – read the details to find out. Some people put their stuff in the public domain, like many of the photos on the sites collected on this public domain image resources page.