Kelly,
Technically, it is 100% illegal to "take" a photograph from a site without permission from the site and from the photographer. The photographer has the rights to the photo until they sell or assign the rights.
With that said, most Internet photos are very small in file size and have limited value past reference materials. The exception might be when someone captures a bunch of images and then tries to sell those images as backgrounds or screen savers. Photos on eBay are normally not of much of a concern since the seller is trying to get rid of the piece. When I needed to use a photo for a web site from an eBay auction, I always write the seller and ask permission. They almost always say yes.
To be really safe, the best way to post a photo here would be to create a link directly to the image on the Internet, vs downloading the image to your computer, uploading to a new server and creating a link to that one. In this scenario, you are simply linking to their photo and not stealing it. Of course, if they remove it, the link will be broken.
You simply have to ask yourself if you would be offended to see a photo you took being used on another site without the person asking? You would probably say yes to most images, but maybe it is a photo of your Grandmother or one of your kids and really don't want to see it used without your permission. Of course, you might be browsing another sign shop site and see a photo of a sign YOU did in their portfolio section. Would that bother you?
In the first two examples you posted above, you are simply showing a photo or scan of some public domain work. The work itself is public domain, but the scan or photo still belongs to the person who took the time to make it. Some universities and institutions are publically funded and display historical images on their sites. You have to read the fine print on their sites, but some will allow you to use them freely. Others require links back to their site or embedded acknowledgments. Almost ALL sites include some sort of copyright or ownership statement, suggesting all text, photos, graphics and images on the site are their property and cannot be used without permission or compensation.
http://www.princetonhistory.org/polImag ... Code=small
This is a link to a page with a photo on the Princeton History site.

This one links directly to the image.
Mike Jackson