Brian, you are not kidding when you say:
"SXC offers free images, but now that Getty has purchased the company, I wouldn’t be surprised if they try to retroactively enforce copyright on all the images SXC has offered for free."
The way to protect yourself against this is to keep evidence that you were offered the images for free. Keep copies of the web sites that were offering them.
Getty will remove old copies of web sites from places like archive.org to make it difficult for you to prove that you were given a valid license for free. Get and keep that proof now if you want to keep using the images. And if you decide to stop using the images, ask archive.org to remove the pages where you have used them because Getty seems to point picscout at archive.org to chase people, as well.
Respect copyrights, but do not respect copyright Trolls.
Live long and prosper!