Where is the line between ethical and unethical usage of images from other Web sites?
Legally
The following is non-expert advice. Please consult a lawyer before following this advice.
Legally, there is a clear line in regards to copying an image. Hosting a copyrighted image in your own webspace is not legal without the permission of the copyright holder. Also, putting material onto the web is not putting it into the public domain. The law is very clear on this point.
When linking to an image, the legality is somewhat murkier. There are two legal arguments:
In Japan and the United states, legal precedent has been set that linking to a page which was not intended to be linked directly (so called deep-linking) is illegal. Images are generally intended for use only within an enclosing html document. Bypassing the document and directly including the image would be covered by this precedent. Note also that this situation fluctuates frequently as the cases involved are settled and resettled.
It's critical to understand that (despite what the FreeSoftwareFoundation says) the current treatment of the laws does not take into account the manner in which a document is constructed, composited, formed, or transmitted; it is the intended representation the intended audience sees that constitutes the document. So, even though the HTML page and the image are transmitted along different sockets, from different servers, they appear both visually and conceptually as one document, and thus they are. Thus, embedding an image without citation is definitely not FairUse.
Also, the fact the original URL is available in the source of the page cannot be construed as a valid citation. You would have to cite the author, date, source. The URL is just where it is stored, which is not the same thing.
Links to specific legal cases would be appreciated.
Ethically
There are a few sides on the ethical repercussions of stealing images on the web, falling on either side of the law.
One school of thought subscribes to the belief that "If you put it on the Web, others can (and will) make copies of it." and therefore copyright laws are not valid. As noted above, this is contrary to the law.
Another is that images should not even be linked without the permission of the webmaster of the hosting site. This is because the cost of bandwidth for the image is being worn by the other webmaster, so you should at least ask first. This view is generally neutral in regards to copyright law, in is more concerned with ensuring that costs are distributed fairly.
And yet another is that images, whether illustrated, or photographs, are artworks. The Graphic Arts community gets quite up-in-arms about it, and its known as "ripping". Generally, anyone trying to become a member of any graphic arts community online will be drummed out if they are ever found ripping. The emphasis here is on the image as art.
See http://www.pirated-sites.com/ for a response to this in the Web Design community.