This has been fixed in the latest version of Snagit on Mac. To download the update, open Snagit and browse to the menu bar item Snagit > Check for updates.
Changing the print orientation in Snagit does not do anything. No matter what it will always print in the same orientation.
This is a bug that we are currently investigating. As a workaround, save the image from Snagit as a PNG or JPEG image, and then open the image in the Mac Preview app to print the image.