We know that very young children enjoy using the pictures to create their own stories. They are not yet ready to understand the ‘squiggles’ (writing) on a page.
As the children become ready to learn to read, they use the pictures to check what is happening in the story to support word solving. That is why the teacher encourages your child to use the pictures for help, e.g. what is in the picture that could begin with those letters?
Children need to become flexible by using a variety of ways to solve words within text. Looking at the picture to gain meaning is just one of those ways.
When the child has the meaning already in his head, he is able to use his time and energy to use the look of the word, e.g. br-an-ch, and ask himself if that word would make sense and sound right within the sentence.
Covering the pictures would make the search for the correct word or phrase more difficult for these readers. Remember that we want them to experience success in order to be enthusiastic about trying new learning.