I just wanted to remind everyone that linking directly to images from other peoples web sites is really rude. This is an issue that's very important to me. I'm talking about on a web forum or on MySpace you just insert an image into the page, but the image is actually coming from someone else's web site.
First, this is just really rude. When you do this, you're loading the image from that other web site. That other web site has to pay to put that image online. By putting it onto a different site, you're asking them to pay for it. That's just rude.
Second, you're breaking copyright law. Most images on the web are protected by copyright. You can't go reproducing them without permission. If you liked the image enough to reproduce it, don't you owe the original creator the respect to not use it against their permission?
Finally, by doing so, the owner of the image can replace it with a different image. Things like this might happen to you: http://www.deuceofclubs.com/switcheroo/.
If you find an image online that you want to share with others, the polite thing to do is to link to the page you found it on. If you're using Google Images to find pictures, Google Images does take you to the page the image was found on. That's probably the correct page to link to.
If you're looking for pictures you can take and reuse yourself, some people welcome you to do so. Check out the "Creative Commons" section of flickr: http://www.flickr.com/creativecommons/. There are over 1.4 million photographs which the creator says you can reuse in any way you want; all they ask is you give them credit.
(This essay originally by and copyright 2006 Alan De Smet. If you see this essay reposted elsewhere by someone else, you can reasonably assume that they chose to post it because they agree with it. This work is licensed under a Creative Commons Attribution-Share Alike 2.5 License. Proper attribution would note that the author is Alan De Smet and provide a link to the original source: http://www.highprogrammer.com/alan/rants/inlining.html )
(Edit 2007-02-20: Liberalize license. Update icon. Minor phrasing tweak.)
(Edit 2007-04-25: "permission." to "permission?")