You can place an image in the background of the chart by using an Annotation. Please see our guide here on creating a CustomAnnotation (X1 Y1 X2 Y2).
My suggestion is to template the BoxAnnotation as in the article above to have an Image at its root.
Next, set X0=XAxis.VisibleRange.Min, X1=XAxis.VisibleRange.Max,
This will ensure the image is stretched across the entire viewport of the chart. If you zoom or pan the image will move with the chart.
Is that what you are looking for? Or not quite the right behaviour? If you want the image to stay fixed always in the background of the chart, check out Annotation.CoordinateMode = CoordinateMode.Relative. You can read about CoordinateMode here.
Let me know if this helps!
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