I am not convinced that he wants a romantic relationship with you. Rather he is boosting his ego by flirting with you and actually enjoy flirting with you.

Your response need to be adequate to the situation. You should respond to amateurism by professionalism

I think calling a general meeting of an hour to set things straight can backfire, because It will sound like a trial to almost anybody. An in a trial, the defendant defends itself.

Rather, you need to cool him down over an extended period of time. Try not to over-think.

He comes to my room very often with no research-related reason and wants to talk, talk, talk about everything but work.

1. Delay. You are in the middle of something important, and he needs to know that. Just tell him that you need to round up with what you are currently doing and that you are willing to go full gossip at the coffee break.
2. Set boundaries. You haven't said what he is talking about. It is mostly about you? Then when it cross boundary you just say it is personal. Dont break under any "Oh come on" pressure tactic.
3. Be a good listener. It may surprise you, but you also have the power to direct the conversation to a subject of your liking. by listening well to what he said you can go deeper in a harmless topic, one which doesnt make you feel uncomfortable.
4. Stop the break when your mug is empty, excuse yourself and go back to work. It is important that all this chitchat happens far from your desk. Desk = work, coffee machine = Small talk of at most x minutes.
5. Invite other staff to the break talk. Don't do it systematically, but sometimes drop by someone's else desk and invite him\her too. It will obviously annoy the shit out of your advisor. Note: At least this work with me. That's where he will show his true color. Either he will invite you offsite, an invite that you can decline, or tell you not to invite other people. That's all you need to hit the ethic institution at your university.

Once, when I was busy working, he came to my room and asked if he could take a photo of me. I felt ashamed and did know that to say and finally, he took the photo.

I am sure the first thing which came to your mind was "Why?". You should probably tell what's in your mind.

Why do you need a photo? It is for the lab website, or a publication? Can we do that at the break or later? Thanks

Sometimes I think he is mad at me that I want to keep this relationship work-only, and then does not answer my emails about our research.

Again we don't have much information about the context. But I will advise you to read How To Win Friends And Influence People. Dont get turned off by the title. It is not a guide to become a manipulative asshole.

Remember that there is politics at the lab and the office. The other lab members are gradually building resentment towards you for your "preferential treatment". You need to take this in account as well.

I am not convinced that he wants a romantic relationship with you. Rather he is boosting his ego by flirting with you and actually enjoys flirting with you.

Your response need to be adequate to the situation. You should respond to amateurism by professionalism

I think calling a general meeting of an hour to set things straight can backfire, because it will sound like a trial to almost anybody. And in a trial, the defendant defends itself.

Rather, you need to cool him down over an extended period of time. Try not to over-think.

He comes to my room very often with no research-related reason and wants to talk, talk, talk about everything but work.

1. Delay. You are in the middle of something important, and he needs to know that. Just tell him that you need to round up with what you are currently doing and that you are willing to go full gossip at the coffee break.
2. Set boundaries. You haven't said what he is talking about. It is mostly about you? Then when it cross boundary you just say it is personal. Don't break under any "Oh come on" pressure tactic.
3. Be a good listener. It may surprise you, but you also have the power to direct the conversation to a subject of your liking. By listening well to what he said you can go deeper in a harmless topic, one which doesn't make you feel uncomfortable.
4. Stop the break when your mug is empty, excuse yourself and go back to work. It is important that all this chitchat happens far from your desk. Desk = work, coffee machine = small talk of at most x minutes.
5. Invite other staff to the break talk. Don't do it systematically, but sometimes drop by someone's else desk and invite him\her too. It will obviously annoy the shit out of your advisor. Note: At least this works for me. That's where he will show his true color. Either he will invite you offsite, an invite that you can decline, or tell you not to invite other people. That's all you need to hit the ethic institution at your university.

Once, when I was busy working, he came to my room and asked if he could take a photo of me. I felt ashamed and did know that to say and finally, he took the photo.

I am sure the first thing which came to your mind was "Why?". You should probably tell what's in your mind.

Why do you need a photo? It is for the lab website, or a publication? Can we do that at the break or later? Thanks

Sometimes I think he is mad at me that I want to keep this relationship work-only, and then does not answer my emails about our research.

Again we don't have much information about the context. But I will advise you to read How To Win Friends And Influence People. Don't get turned off by the title. It is not a guide to become a manipulative asshole.

Remember that there is politics at the lab and the office. The other lab members are gradually building resentment towards you for your "preferential treatment". You need to take this in account as well.