I really wouldn’t worry about it. You’d only encounter problems if… (A) The picture is owned outright by another party and it is exclusive to them. .. AND (B) You are using the image to make money. Great example: Using Dell’s images of computers to sell a Dell computer. They will hunt you down quick and make you stop doing that if you tried.
I can’t really condone infringing on someone’s copyright, but most images are public domain, and there’s tons of copies of just about every pic on the net. It’s just too widespread… after all you don’t check to see who owns a funny pic you want to post on facebook before you upload it do you?
Getty images is one entity that is probably the most likely to cause you headaches if you publish a pic they own and you don’t have rights to it.
But really the only way they’d come after you is if you did that whole trick where you go to istock photo, find a watermarked image that you want, copy the stock number of the image, then go to google images and search that stock number and find a copy of the image on someone else’s site without the watermark that the person was just too lazy to change the file name to something other than the original stock number it came with when they bought it so now you have a free unwatermarked istock photo…..but you won’t do that , will you? No of course not.
Oh, PS – since we’re talking about web images… possibly the best image site on the web that i’ve seen is www.flickriver.com, love that site