How do I get the selected picture in PowerPoint?

This is using the COM (not VSTO) API from my AddIn to PowerPoint.

I have selected a picture. I then do the following:

Selection selection = presentation.Application.ActiveWindow.Selection;
Slide slide = selection.SlideRange[selection.SlideRange.SlideIndex];
// slide.Shapes[2] is the selected picture

How do I know to get [2] instead of [1]? The [1] is a textbox that is on the slide, but is not selected.

thanks - dave

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