Hello Robin,
Thank you for your interest in Syncfusion products.
If your intension is to set background image for diagram model, you need to set model's background style type to texture inorder to show the image. I have tested this condition and it is working as expected. Please see the sample image from the following link -
http://websamples.syncfusion.com/samples/Diagram.Windows/F61447/main.htmPlease make sure that model's background style type is set to texture.
Sample code snippet :
Me.diagram1.Model.BackgroundStyle.Type = FillStyleType.Texture
Please let me know if you still have any more problems with this.
Thanks,
Meera.