He sensed something sinister about the picture but couldn't put his finger on what it was.

put 在这里是动词，意思是"放置、表达"。 his 是代词，指代"他的"。 finger 是名词，指"手指"。 on 是介词，表示"在...上"。 put his finger on 是一个习语，字面意思是"把手指放在...上"，引申为"确切指出、明确说出"的意思，常用于表示虽然感觉到有问题但无法具体说明的情况。