Holy cow, I wrote a book!
Two people pointed out that
the ImageList_Add function
use horizontal strips
even though we learned that vertical strips are much more efficient.
While it's true that
the images are passed to the
functions in horizontal strips,
the image list and toolbar code copy them to a vertical strip.
In other words, the horizontal strip is merely an interchange
Internally, the image list and toolbar
use the more efficient vertical arrangement.