Greetings from Syncfusion.
We have analyzed your query. As of now, we are having support to add a custom image to the chart background only. To add the custom image for chart background, please find the below documentation link,
Right now, we don’t have support to add custom image as chart series fill. But we are having gradient color support to series fill. Please find the below code snippet to achieve gradient color for series fill. Hope this will be helpful.
series.Style.Interior = new Syncfusion.Drawing.BrushInfo(GradientStyle.Vertical, Color.FromArgb(12, 150, 213), Color.FromArgb(117, 138, 150));