The easiest way to achieve what you want is to create a table with a fixed column width and/or row height. Any image you insert into such a table will be constrained to fit within whichever of those dimensions is fixed. If the table is centred on the page, so too will the image be. You can also, if desired, have more than one column so that multiple images can be inserted side-by-side. As with tables generally, tabbing out of the last cell will add another row into which you can insert more images.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]