There are 2 ways:
1- Frame and Image
<Frame CornerRadius="10" Margin="0" Padding="0" IsClippedToBounds="True" HasShadow="false">
    <Image Source="{Binding Picture}" Aspect="AspectFill" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"/>
</Frame>

2- FFImageLoading
<ffimageloading:CachedImage HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"
    Aspect="AspectFill" DownsampleToViewSize="true" Source = "{Binding Picture}">
    <ffimageloading:CachedImage.Transformations >
        <fftransformations:CornersTransformation
            CornersTransformType="AllRounded"
            BottomLeftCornerSize="10" TopLeftCornerSize="10"
            TopRightCornerSize="10" BottomRightCornerSize="10"/>
    </ffimageloading:CachedImage.Transformations>
</ffimageloading:CachedImage>