really nice idea fella, and congrats on doing it yourself, but what nitro says is very valid, whilst mod/admin on another site, i had to close a thread similar to that because of all the bickering.
as for pics, it has to be (i think) 1 pic only per entrant, the thread is opened and each poster puts his pic in with no comments or he will be taken out the comp.
the prize, as you say is done by you so fair play, maybe even modelsport vouchers?.
block voting, not much you can do about it really, were all entitled to vote, not always fair but thats it. one other forum had a guy win an american comp as he got the forum here to all vote for him!!
but good luck with it, like nitro says i dont take that many pics, i just forget, but ill do my best to post some, also if you need help just ask