The two rules as stated:
Images must be 210 px high and up to 375 px wide. This is the
requirement for the header. Please feel free to provide a link to a
larger version, but as explained above, the primary focus is on the
For the same reason, images must be in landscape orientation.
The only firm requirement is that images must be 210px high and a maximum of 375px wide. The requirement for "landscape orientation" isn't needed, but is sometimes noted when someone posts a 375px high by 210px wide image. The second rule isn't needed at all as long as the first is followed, importantly with the correct px high and wide.
As far as I'm concerned, you can post a 210px high 1px wide image to the contest, all the way up to 210px high by 375px wide - the choice is that of the poster. The second requirement should be removed.
More clearly stated the new rule could say:
Images must be 210px high and between 1-375px wide. This is a
requirement for the header.