FAQ 277 has been updated by intrepia to reflect how Snap previews actually work now after Release 39. The new text is
We've partnered with Snap.com to offer link-activated image previews for external links within your journal. Visitors to your journal can preview links and linked images by hovering over the link for half a second.
You can enable or disable these previews at the Viewing Options page. Disabling image previews will prevent you from seeing them in any journal you view, including your own, whereas enabling them will allow you to see them on every journal you visit.
For users who aren't logged into LiveJournal, image previews will display or not display based on the journal owner's settings. That is, logged-out users will see image previews on journals where the owner has enabled them and will not see image previews on journals where the owner has them disabled.