Solved by ptrblck in post #9 
              
              
                Assuming your folder structure looks like this: 
root/
    - boxing/
        -person0/
            -image00.png
            -image01.png
            - ...
        -person1
            - image00.png
            - image01.png
            - ...
    - jogging
        -person0/
            -image00.png
 …