ArcGIS Pro 2.7 API Reference Guide
SetThumbnail Method
Example 

ArcGIS.Desktop.Mapping Namespace > Bookmark Class : SetThumbnail Method
Set the thumbnail for the Bookmark. This method must be called on the MCT. Use QueuedTask.Run.
Syntax
public void SetThumbnail( 
   BitmapSource thumbnail
)
Public Sub SetThumbnail( _
   ByVal thumbnail As BitmapSource _
) 

Parameters

thumbnail
Exceptions
ExceptionDescription
This method or property must be called within the lambda passed to QueuedTask.Run.
Example
Set the thumbnail for a bookmark.
public Task SetThumbnailAsync(Bookmark bookmark, string imagePath)
{
  //Set the thumbnail to an image on disk, ie. C:\Pictures\MyPicture.png.
  BitmapImage image = new BitmapImage(new Uri(imagePath, UriKind.RelativeOrAbsolute));
  return QueuedTask.Run(() => bookmark.SetThumbnail(image));
}
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

Bookmark Class
Bookmark Members