Hello,
I had some difficulties in bringing a graphic to foreground after I had put it in the background. It could not be selected by double clicking or any other means, except by using the Navigator. It should be good to have this mentioned in the guide. I have placed the chapter with the remark in the feedback folder:
http://www.odfauthors.org/libreoffice/english/calc-guide/feedback/0305cg-using-graphics-in-calc/view
Best regards