1.) Don't ask for submissions. This might seem counter intuitive, but clearly there is a dearth of material in game and players are being called upon to support the massive requirement for the nature of the christmas event. Instead of mocking, disappointing, and putting a rift in the community, just do all the work yourselves. Hire staff who are willing to work full time and generate only graphics of quality.

2.)Guide artists to acceptable outcomes Basically don't leave anyone wondering why their art is not acceptable. If there's a player who's pouring hours into creating something for free, there's value there. Even if it takes some time, they can be molded into an asset if treated correctly. That means working with inexperienced artists in order to cultivate their skills and point out where they went wrong. There should be staff assigned to reviewing and critiquing art instead of accepting or rejecting submissions. This goes across the board - uploads getting rejected for no reason creates all kinds of time being wasted. Sometimes hours of effort (for both players and staff) are wasted because a single pixel is out of place or transparent and no one takes the time to explain it to the player trying to upload. It's ridiculous.

3.)Review artist portfolios prior to submission I don't mind drawing. I enjoy it sometimes. But when there's a good portion of time given to any project without hopes of profit, it feels pretty lame to have absolutely no response relating to the effort put in. I spent more than 30 hours drawing for this year's christmas, personally, and have done so in previous years as well. You could prevent a lot of the negative feelings by simply requiring some sort of previous portfolio of work to be acceptable before players were able to submit. This would prevent people with no hope of getting accepted from wasting tons of time, in addition to fostering an environment of sharing and skills-advancement in the community.