What is the difference between them when we use them in the following form:
- This, my picture, is back to 1990.
- This, my picture, is back in 1990.
Your ngram is irrelevant, because it ignores context. As Andrew says in a comment, we use back to 1990 only when there is a (metaphorical) notion of moving back in time.
So we can say this picture goes back to 1990, but if we are not using that metaphor, we have to say in: this picture was in 1990 or this picture was taken in 1990, or this picture is of me in 1990, or this is back in 1990.