In case someone needs this as well, I want to share a simple implementation I made.
Unfortunately there’s no way to display the Image filling the container while keeping its aspect ratio without either clipping it or letterbox’ing it.
This is quite a common task so I created an ImageEx class derived from Image which has an extra Uniform value for Aspect property:
Using other Aspect values Fill, AspectFill, AspectFit works like for the Image class.
You can get a gist for ImageEx class from here: https://gist.github.com/nitescua/c907c2951a719ed3ac34
I wrote a blog post with more info here http://enginecore.blogspot.ro/2015/04/xamarin-forms-image-filling-container.html