That's why - even though it is in portrait format, it still needs to be no higher than 800 pixels (but 768 is better IMHO). So the best way to reduce your picture to keep it in proportion with the original picture, is to reduce it by percentage until you reach a height that is within the accepted range. The width will also be adjusted accordingly, and will be well within the size limit. If you tried to set the height to 1024, or 1200, it would be too large for the screen, and people would have to scroll too far up and down the screen to view it in larger size, and loading would take longer.
"I expect to pass through the world but once. Any good therefore that I can do, or any kindness I can show to any creature, let me do it now. Let me not defer it, for I shall not pass this way again."
— Stephen Grellet