You could be expected by an examiner:
Ø To expand on your narrative using both complex and compound sentences
Ø Apply the right vocabulary to describe what you see
Ø Explain and elaborate on what you see
Ø Explain why something happened
Ø Predict what will happen next
Ø Imagine yourself as a character within the picture that you see
Ø Answer an opinion-based question on an issue raised by the story
Much of this requires sound skills in grammar and vocabulary. Using the right tenses is key to your narrative. Applying the right connectives (and, but, because, hence) will help you to connect your ideas well.
Take the class to put your skills to practice!