This add-in works as described. It should work for new documents, but for existing documents you will probably need to enable it.
With the document opened:
1) Click Add-ons and then Manage add-ons.
2) Hover over the Show picture and select the 3 vertical dots in the upper right corner.
3) Click Use in this document to turn the add-on on or off.
4) Close and restart document if necessary.
This should be a native function in Docs, but this add-on works if you set it up properly.
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