I just tried and cannot duplicate your problem.

The steps I followed were:
1 Created a new presentation
2 Applied "Black" theme
3 Inserted a new slide
4 Chose "Insert Picture from File"
5 Right-clicked the picture, chose "Format Picture..."
6 Chose "Line", color "White", weight "8 pt"
7 At the toolbar chose "Arrange" Rotate or Flip" from the drop-down menu
8 Rotated the image

No dashed line appears. I've done it twice now, even rotated both right and left.

Hopefully you can spot something I've done differently in the steps that will help. If so, let us know what.

Also, does the dashed line show up when you view the slideshow or just when you are in design mode? I've had things show up while I'm designing that don't show when I present.