Images saved under  Templates >  Gallery can be replaced in the text document. Whether the image was inserted on the page or in the text flow makes no difference at this point.

1. Select the image in the mail and right-click.
2. Select  Gallery >  Replace. A list of all other images from the Gallery that have either the same category 1 or 2 will open.
3. Left-click the new image. The format of the new image will automatically be adjusted.
4.  Save your changes.