update readme for responsive images

Author: jwcooper

README.md

@@ -5,6 +5,7 @@
 1. Duplicate `template.md` to `_board/<board id>.md`.
 2. Edit `_board/<board id>.md` according to the template's instructions.
 3. Provide 3 images. An original high-quality image. A smaller image (300 px width),
-and a larger image (700 px width) in each respective directory (assets/images/boards/{small large})
-and process them in something like https://squoosh.app/ to reduce file size.
+and a larger image (700 px width) in each respective directory (assets/images/boards/{small large original})
+and process them in something like https://squoosh.app/ to reduce file size. If
+you only have one image, place it in the 'original' folder.
 3. Create a pull request with the file changes.