Dunno what space you mean exactly but I think you will figure it out once you understand this one thing: flex only affects direct children. In your case you put your images into a div which itself is affected by its parent (Section 15) flex but obviously is not going to be spread out by justify: space between since it is treated as a single element.
In other words either give your div in the section 15 flex parameters and then use it to spread the inner elements or just get your inner elements (images) out of this div and place them into the section 15 itself which has flex property.