Probably a better way to manage this would be to keep the image filenames and their captions in adjoining columns in an Excel workbook. Then, whenever you want to update the collection, do the updates in Excel.

For document generation, you could then use Word's label mailmerge facility, putting the mergefiels for both the image and its caption in the same 'label'. If the data in Excel get updated, simply re-run the mailmerge.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]