My first attempt to translate from Russian to English

Молодец, желаю успехов с английским!

Almost six years ago, in the Summer of 2010, when I was walking along Nevsky with my photo camera, two unknown ('unfamiliar' would sound more natural here) girls come came to me and asked me to photograph them. There were summer, sun, It was Summer, sunny and everybody had was in a good mood. And (Don't start sentences with 'And') I thought "Why not?". I took a photo and keep kept on walking.
When I was reviewing the photos at home, I come across on the girls' photo and thought that it's it was amazing, exemplar (not the right word for this context, *) and that I have had to give it away. But(**) it was would be impossible, because we didn't share contacts contact details. So I upload uploaded the photos in  to Vk and forgot about it. Just in case, I wrote a post in on Vk about this it, however, although I knew that I had no chance to find of finding these girls.

This is very good and makes a lot of sense! However, you have some trouble with verb tenses (come/came, have/had, it's/it was), but nothing serious.

Unknown - this is a strange case in English where the word makes sense, but doesn't work well in context. 'Unknown' implies that you don't know WHAT they are - but you do know what they are, they are girls! 'Unfamiliar' means that you don't know WHO they are - they were strangers!

* Exemplar - This word is only normally used when you are using something as an example in good context. An "Exemplar" essay would be an essay that is so good, people could use it to see how good theirs is in comparison. You can say an 'Exemplar' photo, but this normally means that you intend other people to look at it and learn from it, normally for the technique used in taking the photo. An 'Exemplar' photo would be one with perfect lighting, focus, setting, etc, which you intend for others to learn from. A better word in this context would be 'Perfect'.

** Using 'but' at the start of a sentence. You have to be careful with this - you could say:

"But that would be impossible"

This would work best in your context.

"But, that is impossible!"

This is what someone would say if they were asked to do something impossible, like swimming from London to New York. You wouldn't normally write this.

It is ok to use 'but' at the start of sentences sometimes. 'And' is sometimes used, but it is considered a very bad use of English to do so!