We've actually already paid for art for our first two fiction books.
From the website: "Artists receive shares of gross profit, based on per-project basis"
If you are paying some people up front and making others hope for sales for an undisclosed but probably small share of the royalties that is also an issue. i.e. people the editor knows personally get a better deal?
As an illustrator I have been on the wrong end of that sort of deal long often enough to be very cynical on the matter. So even if there is not in fact any 'some animals more equal than other' stuff it is a not a good look.