This is interesting. When I started blogging, I used images from Google Search. Then I started using images, but linking back to the source as a sort of "by Bob" ass-covering. Now I rarely use images at all, because I worry about this sort of thing.
I do think that Fair Use needs to be extended a little. Using an image posted on Flickr one time, on one blog post, on a blog with almost 1,000+ posts doesn't seem to me like a clear-cut case of infringement. But that's my personal opinion and nothing more.