Hi Michael,
Thank you for your interest in Syncfusion product.
We checked the reported scenario - Unable to remove grouped Picture from PowerPoint presentation at our end. We can retrieve the shape collection of a group shape using the below statement,
IShapes shapes = slide.GroupShapes[0].Shapes;
Please find the sample created to show how to remove a picture from the group shape of a PowerPoint slide,
Sample Link:
Please review the sample and let us know if it could satisfy your requirement.
Thanks,
Arun