Several people have mentioned the Scan-A-Thon. I thought it was a great idea at the time, not just because so many people have lots of old family photos which they really should scan and put up on the appropriate profiles (and a thon does tend to motivate people to do stuff), but also because so many notable profiles lack images, even though there are public domain pictures of them available on Wikipedia (or, more precisely, Wikimedia Commons). But then they announced the rules and said that only photos in your possession that you scan yourself count for the Scan-A-Thon.
So maybe at some point, we could have a Copy-A-Thon (or maybe there's a better name for it -- while do all the thon name ideas seem to start with C or S?), where people getting points for adding public domain images from other sites to the profiles of notables who don't have images on them. (Or, for that matter, images of people who aren't notables, but who have public domain images available somewhere.)
Wikimedia Commons is normally pretty clear about whether an image is out of copyright, or actually in copyright, but being used under the fair use clause. Take a look at the entry for a photo of Wiley Post, for example.
The National Portrait Gallery has a lot of material on their site about re-using images. Take a look at the page for a portrait of Henry Maxwell-Lyte, for example. I haven't contacted them to ask, but I suspect that WikiTree wouldn't qualify as "Use in non-commercial projects (e.g. online in scholarly and non-profit publications and websites, blogs, local society newsletters and family history)." It may be, if Chris is willing, that we could negotiate some kind of site license so that WikiTreers can add any portrait from the NPG site to the appropriate WikiTree profile.
Some archives make images freely available. The Vancouver City Archives holds a photo of Mackenzie Bowell, and it's clearly marked as being in the public domain. (I wish I was in a position to make a donation to the Archives to thank them for being so useful in my research, but I am exceedingly grateful that they don't ask me to pay per use.)
On the other hand, some archives have a set fee for re-use of images they hold. I wanted to use an image of R.E. Gosnell held by the British Columbia Archives, but it says on the page that the image is copyright. So I wrote to the archives to inquire about that. The answer I received said, in part:
While it is true that the copyright has expired for some of our online images – i.e. they are pre1949 images and permission from the copyright holder to make and use copies is no longer required – we hold proprietary and use rights to the images that are in our collection. If you are planning to use the images that we hold in our collection for display on a website - we charge a fee for this use + $20.00 per image for a jpeg file of the image. The fees we charge offset the costs to provide the services requested by our users.
If you are being paid by someone to work on this project, the use fee is $50.00 per image + $20.00 per jpeg file. If this is your own personal project, the use fee is $25.00 per image + $20.00 per jpeg file.
So whether you can use an image from a site without paying for it depends entirely on the site. If we were to hold a Copy-A-Thon (Photo-A-Thon? Portrait-A-Thon? Face-A-Thon?) It might help to build a free-space profile, list the rules for different sites (adding more sites and their rules as we discover them), and then link it under the Genealogy Help category.