Let's say I have a photo of oak in field. I don't know anything about this field. Should I name photo "Oak in field", "Oak in a field" or "Oak in the field" and why ?
Since we can assume you have not referred to these things before (and you say this is the first time the field has been mentioned), all three nouns take an indefinite article.
I have a photo of an oak in a field.
As for naming the photograph, that's entirely up to you. Some photograph (or painting or statue) names drop articles altogether, others use an indefinite article, and others use the definite article.