By default, slide objects are placed on the slide in the order you insert them. If you insert a shape, and then insert a picture — then the picture will be placed above the shape as far as the Z-order of the slide objects is concerned. This sort of Z-order gets complicated when you add many other slide objects. However, it’s fairly easy to control the order by working with PowerPoint’s re-order options. Options to re-order may be found in differing locations, depending upon the version of PowerPoint you use or the operating system you are using — here are links to Re-Order tutorials and resources:
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