Thanks to Eddy_G for providing this tip on dragging an Image into the Editor, rather than clicking the camera icon and doing "Insert Image."
And thanks to Michael_Schoellhorn for providing the prompt which led to this.
Putting this to The Water Cooler for visibility to all users.
1. "Reply" to the a Communities post, and choose your favourite Editor (eg Basic or Advanced.)
2. Open Windows Explorer and navigate to the image file you wish to attach.
3. Arrange both the Communities and Explorer so that you can see both locations.
4. Drag the file over.
I'm still probably going to be using an editor anyway, as I like putting arrows on stuff, but it is useful to know.
ADDITIONAL TIP: If you Edit an existing thread with images, Copy the lot and paste it into a new Communities thread, it takes the images also!
That is how this thread was created. You read it here first.