Each photo you add needs to 'match' the scene in the correct brightness amount.
In any composite image that is made up of many photos, there will usually be a difference in brightness and / or color depending on when and where the photos were taken. It could be in a studio or in the open with sunlight. To make a composite realistic you need to balance the brightness and colour if needed to match. Make it look like it was all captured in the same place and same time.