I find it very important to respect the willing of the owner of photos I use on this blog, partly because I would be sad and angry at people who would not respect my ownership, but also because it is kinder to ask permission, and people are always satisfied when we acknowledge their rights. Sometimes, when I am late and I need to write an article for the day after amd I have no time to wait for a permission to use a photo, I am really embarrassed, like right now.
This is when licenses such as creative commons licenses save my
On some websites, such as flickr or wikimedia commons (the media library of the wikimedia fundation), you can find content licensed under creative commons that you can use on your website without asking for permission. Still, I find it nicer to inform the owner that you use the photo, when possible.
Other such licences exist, but creative commons are really simple. Visit their site for more information: creative commons.