Quote - A bit more info here, such as which is the right text field to use on the submission form (I used the wrong one), and the fact that you can only submit one entry per theme (I submitted two), that postwork is allowed, and that the maximum size isn't in fact 800 pixels on the short edge....

Wish I'd seen this page earlier!

The size is 800 pixels on the shortest side. I submitted an image 1024x768.  That's within guidelines.

It says so here:

Quote - Artwork Submission Specification(s):
Images must be JPEG/JPG format (or compressed into a .zip archive), 800 pixels on the shortest side, maximum 80MB file size. Animation formats accepted are SWF MOV MP3 MP4 WMV, minimum 320 pixels on shortest side, maximum 80MB file size (or they can be compressed into a .zip archive).

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi