I've noticed that several answers, notably Terminology index - a list of bike part names and cycling concepts use images from Sheldon Brown's site liberally and without citation.
Do we have carte blanche permission to use images there? Obviously we can't get permission from Sheldon without using a Ouija board, but:
The pages on Sheldon Brown's Site have various variations of "Copyright © 2005, 2008 Sheldon Brown" in the footer, and sometimes information about a date that John Allen and/or Harriet Fell made some change.
I do not see anything on the site giving anybody else permission to use any material.
So, no, we should not use, except in ways that fall under "fair use".
Perhaps somebody wants to reach out and ask the people now updating the site for permission: http://sheldonbrown.com/contact.html
According to the Stack Exchange Terms of Service, wherein users are referred to as "Subscriber":
Subscriber represents, warrants and agrees that it will not contribute any Subscriber Content that (a) infringes, violates or otherwise interferes with any copyright or trademark of another party, ... (c) infringes any intellectual property right of another or the privacy or publicity rights of another...
So no one should be using images in a way that violate another's copyright or trademark. According to the same TOS, any user who does so risks termination or suspension from the entire Stack Exchange network.
Stack Exchange may also terminate or suspend any and all Services and access to the Network immediately, without prior notice or liability, if Subscriber breaches any of the terms or conditions of this Agreement.
All of that said, any user who uses images in a manner that is considered fair use is not infringing on copyright or intellectual property rights. I will, however, admit that I find fair use doctrine to be a muddy and messy affair. My gut says that the vast majority of images that we use constitute fair use. My reading of what constitutes fair use makes me think that it may be borderline. A definitive answer would have to come from someone more legally inclined than myself.
As to the second part of your question, whether or not to cite (just my opinion, so mod hat off): absolutely. Legality aside, it's just the right thing to do.
My perspective comes from 20 years of Academic work, where the fair use and also ethical behavior boundary is often tested, and by my experience as an expert witness in international copyright cases.
As @jimirings has pointed out, the terms of service are clear.
I mentioned in a comment
Yes, I think it is fair use. But I also think they should be credited.
To me the right way is for credit to be given in the form of a link to the originating site. That way, they potentially get traffic from our visitors, and they also get a higher Google page rank.
When I get a little time, I'll knock up a script to find all the images.
See http://data.stackexchange.com/bicycles/query/372048/posts-containing-png-or-jpgs (now revised to show View counts). It shows there are 2387 as of the last Data Explorer update. I tried adding
, CASE WHEN Body like '%http://www.sheldonbrown.com/%' THEN 'Yes' ELSE 'No' END AS Sheldon
to the SELECT statement. It doesn't help much - only 19 of those 2387 have a link to Sheldon, and the ones I checked didn't use images from Sheldon's pages.
After we've found the images, we could use a Google image search to see if they are matched elsewhere. I think we'll need to design a process so that any work is coordinated, since each post would need to be manually checked.
One idea is to serve them onto a new review queue, and provide bronze, silver, and gold badges.
