take a picture或take some pictures, 前一个是单数,后一个是复数。To take pictures 给人照相.To have pictures taken让人给自己照相.“他们正在拍照”用英语应该这样翻译:"they are taking pictures."take a picture of/for sb. take a photograph of sb. photograph sb ,photograph和picture作名词时都是可数名词,直接加s.take photos of sb:为某人拍照photograph sb:为某人拍照take photos / take a photo