Let's suppose you are listening to a conversation between two Japanese speakers and you don't speak Japanese at all. What the Japanese speakers could conclude about you?
They could come to the conclusion you haven't got the picture about what they were talking together.
This post's idiom is get the picture. It means "to understand a situation".
Let's imagine a few more examples:
It was coming so much news about the new president measures, that at first, I couldn't get the picture what was happening;