Hi Drew, Mail templates are made using HTML (Web page formatting) so a “Pages” design won’t export in the proper format.
The easiest way to solve this is to open a fresh email form, add an image or logo in the space below where your text will eventually be placed, add “Salutation” at the top with any universal wording e.g.
“Dear Member,
here is next month’s news…”
Format to taste and then under Mail’s “File” menu, “Save as Stationery”.
Your new pre-formatted email will appear in the “Custom” section under the Stationery Icon in the message top menu bar. If you don’t see it there you can add it by going to the “View” menu and clicking on “Customise Toolbar”.
Then you type whatever you wish within the blue selection border, and send.
I have sent you some screen shots of the process.
There are a number of almost free apps to edit small images, but if you need an inexpensive but very flexible and effective app for making and editing images you might like to look at “Acorn 5” in the App store (currently on offer at £18,99) - Yosemite or later.
Much more sophistication comes with Affinity Photo, and/or Affinity Designer (£39.99). All have a free trial period. We are scheduled to look at these during the October Dorchester meeting.
It appears that Mail does not like a graphic on the same "line" as text so just open your Pages document, use Shift>Command>4 to do a screenshot of the complete heading (text and graphic). Drag the resulting saved graphic (from your desktop?) into a new blank signature.