https://learn.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.media.imaging.bitmapimage.imageopened?view=windows-app-sdk-2.0
BitmapImage.ImageOpened Event (Microsoft.UI.Xaml.Media.Imaging) - Windows App SDK | Microsoft Learn
Occurs when the image source is downloaded and decoded with no failure. You can use this event to determine the size of an image before rendering it.