Using people's pics online

At my talk about blogging last Tuesday, Paul Herrmann from Redeye www.redeye.org.uk asked where the law stood on lifting people's images to use on blogs. I said I wasn't sure, but couldn't see a problem if you were praising someones work and showing it to a wider audience. Surely as egotistical artist types, we want as many people as possible to see our work? It appears that at least one person disagrees, read more here:http://fetching.net/2007/10/please-dont-steal-my-work/