I have recently received comments that I used the photographs of another photographer without his permission.
Here is what happened. I received the photos from a third party. That third party had asked the photogrpaher if the photos could be used on my blog. He had e-mail permission from the photographer to [quote] "do with them what you wish".
To my mind, that is permission.
However to avoid ill-mannered (and of course anonymous) comments in future, I will use only my own photos or those in the public domain through creative commons or other sources.