How can I draw a Polyline onto an Image (WPF)

You do not need a rendered Canvas or any other visible Panel at all.

Just use basic drawing primitives available at the Visual layer.

The DrawGeometry method below draws a Geometry onto a BitmapSource, using the bitmap's rendered size, i.e. the size that takes its DPI into account, and returns the resulting BitmapSource.

public static BitmapSource DrawGeometry(
    BitmapSource source, Pen pen, Geometry geometry)
{
    var visual = new DrawingVisual();
    var rect = new Rect(0, 0, source.Width, source.Height);

    using (var dc = visual.RenderOpen())
    {
        dc.DrawImage(source, rect);
        dc.DrawGeometry(null, pen, geometry);
    }

    var target = new RenderTargetBitmap(
        (int)rect.Width, (int)rect.Height, 96, 96, PixelFormats.Default);

    target.Render(visual);
    return target;
}

In order to draw in the bitmap's pixel units and hence ignore its DPI, modify the method like this:

var rect = new Rect(0, 0, source.PixelWidth, source.PixelHeight);

using (var dc = visual.RenderOpen())
{
    dc.DrawRectangle(new ImageBrush(source), null, rect);
    dc.DrawGeometry(null, pen, geometry);
}

The following method uses the above to draw a polyline as an IEnumerable<Point>.

public static BitmapSource DrawPolyline(
    BitmapSource source, Pen pen, IEnumerable<Point> points)
{
    var geometry = new PathGeometry();

    if (points.Count() >= 2)
    {
        var figure = new PathFigure { StartPoint = points.First() };
        figure.Segments.Add(new PolyLineSegment(points.Skip(1), true));
        geometry.Figures.Add(figure);
    }

    return DrawGeometry(source, pen, geometry);
}

It would be used like

var source = new BitmapImage(new Uri(...));

var pen = new Pen
{
    Brush = Brushes.Blue,
    Thickness = 2,
};

var points = new List<Point>
{
    new Point(100, 100),
    new Point(1000, 100),
    new Point(1000, 1000),
    new Point(100, 1000),
    new Point(100, 100),
};

image.Source = DrawPolyline(source, pen, points);