custom.cs

public class CustomImageRenderer : ImageRenderer
    {
        MyRect imgBorderRect;
        private Rect mBounds;
        private Paint mBorderPaint;

        protected override void OnElementChanged(ElementChangedEventArgs<Image> e)
        {
            base.OnElementChanged(e);
            if (e.NewElement != null)
            {
                var img = e.NewElement as CustomImage;
                imgBorderRect = img.BorderThickness;
                mBounds = new Rect(Control.Left, Control.Top, Control.Right, Control.Bottom);
                mBorderPaint = new Paint();
                mBorderPaint.StrokeWidth = (float)imgBorderRect.Left;
                mBorderPaint.Color = img.BorderColor.ToAndroidGraphicsColor();
            }
        }

        public override void Draw(Canvas canvas)
        {
            if (imgBorderRect.Left != 0)
                canvas.DrawLine((float)(mBounds.Left - imgBorderRect.Left / 2), mBounds.Top, 
                    (float)(mBounds.Left - imgBorderRect.Left / 2), mBounds.Bottom, mBorderPaint);
            if (imgBorderRect.Top != 0)
                canvas.DrawLine(mBounds.Left, (float)(mBounds.Top - imgBorderRect.Top / 2), mBounds.Right, 
                    (float)(mBounds.Top - imgBorderRect.Top / 2), mBorderPaint);
            if (imgBorderRect.Right != 0)
                canvas.DrawLine((float)(mBounds.Right + imgBorderRect.Right / 2), mBounds.Top, 
                    (float)(mBounds.Right + imgBorderRect.Right / 2), mBounds.Bottom, mBorderPaint);
            if (imgBorderRect.Bottom != 0)
                canvas.DrawLine(mBounds.Left, (float)(mBounds.Bottom + imgBorderRect.Bottom / 2), 
                    mBounds.Right, (float)(mBounds.Bottom + imgBorderRect.Bottom / 2), mBorderPaint);
        }
    }