Hello Kevin,

Sorry for the delay. We have added the MarginAspectRatio property to the ImageAnnotationBase class (it's new abstract class base for all annotations with images) in new version of VintaSoftImaging.NET Library. New version will be available at the end of this week.
Also we have updated ProportionalImageAnnotation class and "MarginAspectRatio" functionality will be available in custom annotation.

Best regards, Alexander