How to re-enable preview images in Trending section on Facebook?

Well, since I have it - and tooltips or link previews are pretty standard, I'm thinking that you might want to do the ole spyware sweep (it never hurts to do such things).

Recognize that Facebook is the biggest A/B test in the history of software and online testing, so sometimes people see different versions of the site. There's a chance that that is what's happening with you although I'm thinking that spyware is a more likely culprit.