To do that, you would need to put a single-cell table on each page, in the location where you want the image to go, then give the table a fixed width (i.e. set the preferred width and disable the 'automatically resize to fit contents' option) and set the row height to an 'exact' size. Then, whenever you insert an image, insert it in-line into the table. Thus inserted, the image will resize to fit the available space, with the correct aspect ratio.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]