Photato(Photogenic Potato) refers to someone that looks good in a photo, but looks ugly in person. 

上镜的土豆指的就是那些本人长得并不出众，却很上镜的人。

She might be someone with a small face, or a slim body, or someone who is relaxed with the camera. All those factors could make her look more beautiful in the photo than she actually is, which makes us jealous.

也许她拥有一张小脸，也许她身材苗条，也许她面对镜头很放松……这些都能让她镜头上比本人更漂亮，也让我们更嫉妒。

Example:

That girl is such a photato. If only she was as good looking in real life, as she was in her Facebook photos. 

那个女孩就是个上镜的土豆。要是她真人和她Facebook上的照片一样好看就好了。