I was also playing around with that and at first I also was not quite sure and had to try different things.
But it is like Sylvain/the docs say: instead of putting in a list of points with x,y values (x = horizontal, y = vertical coordinates) for the four edges you just give them the bounds, i.e., upper y (= Top), lower y (= bottom), left x (=left), right x (= right):
| = left
┌───┐= upper
│ │
└───┘= lower
| = right
Hope this visualization helps you to see some nice bounding boxes! 
EDIT: Ok, you solved it later here ImageBBox.create Error - TypeError: slice indices must be integers or None or have an __index__ method