[CW]

When I read this thread this afternoon during lunch, I was in the horizontal camp. Now, after Andy reiterated about the odd tilt effect that the T206 artists sometimes employed, and after 10 minutes of playing around with photoshop, I can conclude that the original photo was most likely, almost definitely, a vertical shot of Jack Dunn getting ready to make a basket catch. The key was getting rid of the tilt effect, as shown in the Chief Bender card by Andy.

All I did was isolate Dunn from his T206 card, then I rotated Dunn clockwise maybe 20-40 degrees until his belt was basically horizontal (or at least until it looked normal) -- this removed the tilt. Due to the cropping of the T206 image, I had to (crudely) draw in a little bit of pants below Dunn's belt in the lower left, and also a bit of Dunn's elbow.

Once I had this image, I could view it both vertically and horizontally, and I think the the image looks MUCH more natural in the vertical position, as if Jack is getting ready to lower his hands to make a basket catch, as others have mentioned previously. When viewed as a horizontal, it just doesn't look right, and the arms simply don't look like he's naturally diving.

The first image with black background is original from the card, the 2nd image is vertical without the tilt, and for the 3rd image I simply rotated the 2nd image 90 degrees.