Stealthie is a photograph of one or more people, taken without their knowledge or consent.

未经别人同意、或者在别人不知情的情况下拍摄的照片叫“偷拍照”(stealthie)。

Smartphone cameras are not just for selfies, they're also for taking “stealthies,” or secret snaps of people around you, mostly in the “OMG, look how hot this guy/girl is” category (admit it, you and your friends have a Viber/Whatsapp/WeChat thread dedicated to this).

智能手机自带的相机并不只是用于自拍，还可以用于偷拍，也就是偷偷拍你周围的人，大部分偷拍都属于“天啊，快看帅哥/美女!”系列(承认吧，你和你的朋友们的各种朋友圈里都有这一类内容)。

For example:

There are 468 24-hour security cameras in Cannes, which means, even if you aren't taking your own picture, someone else is. Think of it [as] a “stealthie.”

戛纳一共有468个24小时监控摄像头，也就是说，就算你没在给自己拍照，也有人在拍你。就当是被“偷拍”吧。