When this grammar point comes up in class, I say "When I say 'the', I know that you know, or I think that you know, which one I'm talking about. This happens for three reasons: 1) I've told you before, 2) I'm telling you now, or 3) I'm talking about the only one or the obvious one, here, now."
"I've told you before" would happen if this picture was part of a series. In a previous picture, or earlier in the video, we have seen the mother, the baby and the cot/crib and read the words "Here is a mother with a baby. She is putting the baby into a cot/crib".
"I am telling you now" would happen if I said "The mother in this picture is putting the baby in this picture into the cot/crib in the picture", which is obviously awkward.
That leaves "the only one, the obvious one". There is only one mother, one baby and one cot/crib in this picture. I don't have to tell you that - you can see them. I am talking about "the only mother", "the only baby" and "the only cot/crib". But there is also only one picture on the wall, and she is putting only one blanket over the baby, so why don't we say "There's the picture over the cot/crib" and "She is putting the blanket over the baby". Firstly, a baby and a cot/crib are integral to a baby's bedroom, while a picture (more) and a blanket (less) aren't. Secondly, the pattern "There's ..." is more often used to introduce "a [something]" rather than "the [something]". "She is putting the blanket over the baby" is much more possible, because of the sentence pattern.