Hi Kurt,
Thanks for using Syncfusion products.
The “Opacity” property of watermark values only work for watermark text. It doesn’t work for watermark image. You can find more information about watermark properties in the online sample link from below
http://asp.syncfusion.com/sfwinrepsamplebrowser/9.4.0.62/Windows/Chart.Windows/Samples/2.0/Chart%20Appearance/Chart%20Watermark/Sample.aspxPlease let us know if you would require any assistance.
Regards,
Vijayabharathi