@billstevenson The image has to be on a server somewhere (dropbox etc.). Click on the postcard looking icon when composing a message and provide the link to the image file when the pop up window is shown. This blog doesn’t seem to like large images so to prevent it from being distorted, resize it in the pop up window to be 600 px wide or less.
EDIT: I just saw that elliottbnewcombjr posted as I was typing and his explanation is much better than mine.

