I found at least three different stories for this picture.
The first is that the soldier carries the donkey on his back because it is in the nature of a team member to help others in need.
The second suggests that they are in a minefield and that if the donkey were free to move around, he would quickly blow up the whole team.
The third says that it is always better to lead a donkey than to follow it.
Whatever the story, I have only one moral: when there is a donkey in a team, you will soon find it on your back.