Using 2014.1.403. In the image properties dialog, the icon for the option to constrain the image aspect ratio is missing. The browser loads the icon image from http://d35islomi5rx1v.cloudfront.net/ajaxz/2014.1.403/Common/CommonIcons.gif
. The section with the constrain icons is empty. Your demo site pulls the image from a resource, and it is complete.
As a workaround, I took a local copy of the correct image, and added css to SetImageProperties.aspx to use my local copy.
I also modified this file to change the default setting of this button to constrain the size, which I think is more useful to users.