Images can be 1400 pixels wide and 1050 pixels high.
Here are some examples of images which can be submitted:
a landscape which is 1400 x 1050
a vertical which is 800 x 1050
a square image which is 1050 x 1050
Note that the vertical dimension cannot be more than 1050.
If you accidentally forget to resize your images, Photo Contest Pro may still accept them if they are not too far off.
However, everyone should strive to resize their images to conform to the guidelines.