I have seen a couple of entries in portrait orientation, however when I try to upload one I get an error as it is over 600px high.
Is there any way to do it?

For Non-Advantage: The image must be no more than 800 pixels wide x 600 pixels high and no less than 500 wide x 350 high.

So, 500 pixels wide x 600 pixels high is the "tallest" image that fits the requirements.

If your portrait image is, let's say 300 pixels x 600 pixels, add white margins on each side to make it 500 x 600. The white will blend with the site background.

See http://photoshopcontest.com/view-entry/213447/mrs-robofone.html

thanks for the advice