CGSociety, aka CGTalk, used to run a yearly themed illustration challenge. I frequently participated (and was sometimes a judge.) Artists were active in the online forums, giving each otehr feedback on their entries-in-progress as we all developed ours. It was a terrific expereince and I made a lot of artist friends that I am still friends with to this day.
Here are several of my entries.
'Adorable Alien,' Uplift Challenge, Excellence Award
'Farewell Kiss,' 'The Journey Begins challenge, 4th place
'Wage Slave,' Master and Servant Challenge, honorable mention
'Wandering Monsters,' Strange Behavior challenge
'Valley of the Vampire Vixens,' B-Movie Challenge, winner - Best Concept Art
'Escape Plan B', Steampunk Challenge, honorable mention.