• Create an label containing a large image (e.g. 2000x2000px).
• View the label in the mobile app.
• The image is constrained by the width of the screen/window, but is the original length rather than being scaled down at the original aspect ratio.

Adding height: auto; to the .atto_image_button_text-bottom.img-responsive and .atto_image_button_left selectors seems to fix the problem.