|This image brought to you by Wikipedia Commons.|
Totally legal to use, and totally flipping creepy.
Henceforth, expect similar creepiness and irrelevance.
It doesn't matter if you go out of your way to list the source from which you borrowed a photo, you can still be sued for breaking copyright laws. Half the time when I use a photo on my blog, even if I'm citing a source, I don't know for certain that the source I'm citing is the owner of that photo.* It's like a telephone game of image borrowing, and who the heck knows where it originated? It doesn't matter if I cite my source. If I used it, I can be sued.
Again, it doesn't matter if my blog is not-for-profit. If I use somebody else's image without their say-so, I can be sued despite not making any money off of it. For many artists, it's the principal of the matter, not the loss of potential profits.
Providing a Disclaimer
Just because you went ahead and added a disclaimer to your blog doesn't mean you're in the clear. Stating that you are not claiming ownership or rights to the image doesn't absolve you from copyright infringement or from any repercussions.
Unless you receive permission directly from the owner of the copyrighted image, you are breaking copyright law and could end up paying the price. Roni Loren herself ended up finding this out the hard way. The only 100% safe images are those gleaned from sources providing approved images for public use, or those you take yourself (which could be a lot of fun).
As for Pinterest...
Take a moment to read their Terms & Privacy section. It's enlightening, I assure you. In short, it places the burden of responsibility and any ramifications of copyright infringement on you the pinner. I don't know for certain, but I'm betting the same rules apply when pinning copyrighted images on Pinterest as using them on your blog. Do so at your own risk.
I guess I'm just seriously asking myself: Is it worth the risk? As a writer hoping to get published, I of all people should be wary of borrowing content without permission. How would I feel if the borrowed content was from one of my stories? As much of a pain in the arse it might be, I'm going to try and only use my own photos or images in the public domain or creative commons from now on. I might not find exactly what I'm looking for, but at least I won't be breaking any laws.
If you blog, tumbl, or pin you should really take a look at Roni Loren's article for a more comprehensive look at this topic.
* Okay, probably far more than half the time.