This is taken from the theme review guidelines:

"Themes are required to be 100% GPL-licensed, or use a GPL-compatible license. This includes all PHP, HTML, CSS, images, fonts, icons, and everything else. *All* of the theme must be GPL-Compatible."

I don't see how an image released under an Attribution-NonCommercial license is GPL-Compatible.

If the image (or any other resource/part of the theme) was GPL compatible, people would have the freedom to include it in both commercial and non-commercial works.

They don't have that freedom due to the images NonCommercial license.