If you are just thinking of images for a blog, I have been 'borrowing' images for years and ...knock on wood, have had no problems. But if you want to use them on permanent pages, do not mess around. You will get nailed. This isn't like anonymously downloading a movie. These images are online for all to see, including image recognition robots. Same tech that tineye uses.
Most of these are permanent licenses. Some sites have some classification of images that aren't permanent. Some say you can use on web but have to pay separate for print or vice versa (but non web is pretty hard to enforce)
I think this is one of those luxuries, 90 minds should pay for. If folks are too lazy and stupid to blog, that's there problem, but we should be able to finally get a benefit for all the extra effort we put in vs. the other members.