Hmm.. Y'know, I like this contest idea, though you might want to elaborate more on what you want to be submitted and what exactly you intend to give as a price?
Let me try to summarise it, ok?
Inversed Roles Contest:
-You must submit fan-artwork depicting heroic deceptibot Megatron or vile autocon Optimus Prime, basically the inverse of their roles as autobots/decepticons.
-The winner gets to choose as a prize from either a custom repaint of an alternator, their entry kitbashed orperchance a kitbash of choice if their entry would cost too much to KB.
-Why limit it to these two leaders?
An inversed Starscream or Blaster might be just as neat, eh?
As for on how to host the contest, maybe these tips will help:
Decide on the rules beforehand:
-Make a minimum/maximum picture size limit.
-Decide on who/what group will be judging the entries.
-Mention that the contest follows the forum rules.
-Be sure to post a picture of Reverse Convoy/Megatron so people don't send in artwork of that toy by acident.
-Set a time-limit, preferrably let the contest run along for some 3 weeks.
-Take no more then a week after the contest ends to conclude judging.
-Remind people that one person=one entry and no more.
Decide on the Prize-budget:
-Decide how much you would be willing/able to spend on the purchase of the Alternator/kitbash.
-Do some research on the costs of Alternators/base bodies and then make a mention on which bots/cons would be too expensive to KB or even repaint.
-Make mention of earlier kitbashes by your hand, so people can look these up if they like.
-Offer an option for a gift certificate(to some TF selling online store.(not all live in the u.s. of a.)) in case the winner really doesn't want a kb/repaint yet did enter the contest.
Other things to consider:
-Whomever does the judging cannot enter the contest, so ask them before starting.
-Consider purchasing a set of custom heads.
-To save money, consider these quality BT-Ko's, you can find them in this ebay shop.
-Consider refraining from allowing only handpainted/drawn entries, not all have scanners.
-Last, but not least, consider making an image-header for your contest, text-alone contests seem to be less popular.