Ok, just some thoughts of mine. 

Goals of the Art Challenges:   

• To add more art to the site

• To grow artist’s skills

• To build the community
• To add motivation and inspiration to new artists
• To assist specific games with art needs

Variety of Art Challenges:

• Weekly Challenge       (weekly basis and time frame)
• Monthly Challenge     (monthly basis and time frame)
• Co-op Challenge             (two partners teams for each contribution)
• Team Challenge              (team participation for each contribution)
• Art Sprint        (limited to a short designated time and focused content)
• Game Specific        (contributions limited to a specific games needs)
• Art Dual             (one-on-one challenge voted on by members)

Format:

• Dedicated page
• Every entry for current challenges will have preview on the dedicated page
• Challenges in voting stage will be shown as previews below current challenges
• Winners of past challenges will be shown as previews below voting section
• Provide email and feed support, notification is desired
• Automatic updates   (allow me to prepost in some way?)

Other possible ideas
show number of challenges entered for each member
an “i voted” medal

What might be issues keeping the challenges from succeeding?
artist doesn't have enough time
didnt know the latest topic
not interested in the topic
not enough excitement about contributions
doesnt have enough skill

So how can YOU help?
Participating in the challenges!  :)
Do you know of any like-minded sites where the challenges could be posted?
Would you be interested in helping to prepare a color palette or guidelines for a challenge?

Very nice work!  I had to play it several times.  :)

Tomorrow is the final day for this competition.  Good luck everyone!!

nice, are you going to add textures soon?