First I would turn on the Rulers, Guides, and Snap to Grid (but not the actual Grid) under View > Show/Hide Rulers & Grids. Pull a guide out from the left hand side to the center of the page and one from the top to the center. Next, activate edit mode by selecting the Edit Text & Images command under the Content Editing panel. Select your picture and you will see small selection boxes in the center and sides. Align these to your guides.
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