What is the correct method to display then save an image file ?

Hello,
I am quite new to the Xamarin world I am trying to realize simple exercice apps to discover Xamarin forms. For now, I am trying a simple thing: I have an xaml with something like this:

<Image HorizontalOptions="Center" Source="https://<anypathtoanimage>" x:Name="mainPic">
                <Image.GestureRecognizers>
                    <TapGestureRecognizer
                Tapped="TapGestureRecognizer_Tapped"
                NumberOfTapsRequired="1" />
                </Image.GestureRecognizers>
            </Image>

And from there, I thought "Ok, let's tap the image and offer to save the displayed image somewhere".
Well, I understood how to save a file on the local storage (thank you FilePicker), I did some tests on text files etc...
From here, next step is to retrieve the data from the downloaded image. My first thought was to simply retrieve the corresponding array of bytes which must be stored somewhere in memory. I'm stuck here. I've tried to find something about caching, but still hit a wall.
Is it possible to do something like this while having my image loaded directly from xaml like I did, or do I have to load it first from code, and add an intermediary step where I manually save the data ?
Any idea appreciated, maybe I'm all wrong with all this in my approach, I'm all ears