This raises two questions in my mind:

1: How do you verify an image really does conform?

2: How are people creating these images? Obviously not by hand, with 16,777,216 individually coloured pixels involved.

Some of the submissions link the software used in the description.

2²⁴ isn't too terribly large, so there's a few possible validations: iterate and set bitflags for each color already seen, sort the pixels and check for duplicates, ...