When you insert them, they default to a 'Square' wrapping, which permits you to position it anywhere on the page, but you can convert the wrapping to in-line via the 'Wrap Text' button on the 'Picture Tools' tab while the shape is selected. If you do that, the autoshape will assume whatever alignment the underlying paragraph has, so you might want to attach it to an otherwise-empty paragraph for complete alingnment control.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]