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.