The meaning of 照相 is taking photos or taking a photo.And the meaning of 照张相 is taking a photo,only one photo.

照相 - take a picture 一张相 - one photo , 张 is a measure word for a picture 照张相 = 照（一）张相 - take one photo, when number is one, Chinese people like to remove it.

照相 is a general verb for taking pictures, like in " 他喜欢照相。“ 照张相 is a shortened verbal phrase for "照一张相“，which is the actual action of taking a picture, it's a colloquial expression, like in " 我们在这儿照张相吧。”

一个是动词加名词 一个是单纯的动词

“照张像”是“照一张像”的简单表达方式，“一张”相当于“a”，“一”是数词，“张”是量词，这句话省略了数词。 例如：“吃一个苹果”，可以说成“吃个苹果”，这样表达更地道