Sets high resolution icon for Retina mode on Retina displays running on Cocoa
This high resolution image has to be exactly 2x larger than the main icon.
If having a high resolution icon then you must also have a icon else no icon is displayed. End size is controlled by the main icon.
In newer versions of Xojo you should not use this property. Instead make Image sets with up to 3 resolutions and pass the image set to the Icon property.