Drag it to the desktop (for example) and then double click on the image to open it in Preview. From within that app under Tools select Adjust Size and then set the height to 150 pixels. If the width is 1200 or more at that point, it's good to go.
To upload for banner consideration, go to this page
https://garden.org/barn/banner...
Select "Choose File" at the bottom then upload and preview.
If any of that does not make sense, I can try to clarify.
