Here is xaml (an image in stacklayout). Everything is logical: I set Aspect to AspectFit and HorizontalOptions to FillAndExpand. Width of image must fill the width of stackl
Up until now, the only solution I've found is to set Aspect="AspectFill" and set HeightRequest.
You can bind HeightRequest to a property and calculate the height based on the page width.
In my case the images are always the same proportion, so I do:
HeightRequest = (int)Math.Truncate(Xamarin.Forms.Application.Current.MainPage.Width / 1.41);