"You are a nice girl" wouldn't refer to your appearance, but more likely to your personality. If a person was saying you look nice then they would say it.

We can also say, "you seem nice". 

But, in the English speaking world most teachers, writers, journalists, politicians etc hate the word nice. It's so boring and has too many possible uses. Generally, we prefer to use other adjectives.

Hm, i think he means that you have a good heart inside your body. And outside you are beautiful. 

Thank you so much for your help :) I myself don't like the word "nice" too, hehe

Hello Gertie! Thank you for your help. Hmm in the first case I agree with you. He might mention the conversation. In the second case, it's a female friend of mine. We had been talking to each other and it was enjoyable, before I showed her my photo. I wonder if she wanted to tell me that I'm a nice girl which is similar to the first case, or My appearance is nice. :) 

Nice usually means sonething along the lines of "kind", "good", or "pleasant".

I don't know who your new friend is, but that's weird.

The first friend's message made perfect sense. He meant that you seem like a kind person. He could also have said something like "Our conversation was nice" or "Talking to you was nice." In this case, it would mean "good" or "pleasant".

The second friend is a native English speaker? It is really a female? Perhaps they thought something about your appearance looked gentle or innocent. I'm not sure why she said that ^^