By the letter of the law, any original created work is copyrighted, even if it doesn't say so. That includes both text and images.
So that means that if you include an image in your blog, you are copying it, and you have to have rights.
Note that putting in a link to an image can be interpreted as copying, and even if it isn't, some people don't like it; it can also be a big impact on the hosting site for the image, see Tim Bray's blog about this.
Many Dear Readers probably already knew about this, but there is an entire site called the Wikimedia Commons which dedicated to media that you can safely copy, normally licensed under GPL or Creative Commons. The other thing I do is use Flickr, and search for images that have the right Creative Commons license.
By the way, if you ever want to include some of my Flickr images, go right ahead, I've licensed them using basic "Attribution Creative Commons" -- all you need to do is include an attribution when you include. Although I don't know why you'd want to include pictures of my baby son or grainy pictures of Prague.