Most likely, whatever site attached the coupon formats the print job to print in landscape orientation. If you are able to open the file and change the orientation to portrait, then it should print in portrait.
Depending on the file type, image or document, the changing of the orientation may not be possible. In your reply, you specified it is an image file, which most likely will not be customizable to print to read in a portrait orientation.
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Although I work for HP, my posts and replies are my own opinion and not those of HP.