I'm not an expert on the law (though I have spoken to a lawyer). I try to use my own photographs (I take a lot specially) or those gathered from Wikis with a clear statement that they are in the public domain. Some other sites specifically allow use on blogs (though they may insist that the blog doesn't carry advertising, which is something to look out for.)I use cover images and other promotional material on the grounds that the copyright owners are trying to promote these things so they'd be nuts to limit their use (provided you are being positive about the thing being promoted). I always ask guest bloggers to confirm that they are not using copyright images. Often they don't bother, which is annoying.
I will use images that include me, whether or not they are copyright. If they want to sue, I'll sue back for unlicensed use of my image.
Just be careful and don't merrily use any old images. (I put a copyright note on most of my own now, because I'm fed up of people stealing them.)