It would be possible to have a dropdown content control with text (e.g. the image name) that, when selected, gets replaced with an image from a file on disk, but you couldn't then choose a different item from the dropdown - since it gets replaced with a different kind (e.g. 'picture' or 'rich text') containing the image. Given the complexity and inflexibility, you may as well just use a picture content control and let the user select the image from it's dialogue box.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]