To constrain your pictures to fit within a given size, create & copy a single-cell table with a fixed height and width to wherever you want the images to go, then insert the field code into that. Any picture inserted into it will have its dimensions constrained to fit the space available, at the correct aspect ratio for the picture. To set the fixed:
• row height, use Table Tools>Layout>Properties>Row>Specify Height>Exactly.
• column width, first uncheck the 'automatically resize to fit contents' option under Table Tools>Layout>Properties>Table>Options. Then set the preferred column width under Table Tools>Layout>Properties>Columns.
You might also want to set the cell margins to 0 all round. It's also a good idea to format the cell paragraph with 0 before/after spacing.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]