Wow, that's incredible idea Felix! You are very creative!
I think it worth having pictures painted within competition time frame and then give extra week after event is over for voting. I suggest to think about having voting at the last week of the event plus one extra week.
Time is from 4 weeks probably to 6 weeks.
I think it will be challenging to get sponsors that are available everywhere in the world- maybe Starbucks, macdonalds, Amazon, etc.
Voting: this is a tricky question. I think 3 votes shall be given per member of the site. They shall not be deposited to the same person/painting to give wider pool of voting. Votes may be they can be granted to someone outside of the membership as well. This will allow to attract more viewers and potential members. It also depends how many time you have to maintain the process going as I think jury from sponsored events also can be an option. But this is time that's needed from you and from jury to go through the paintings.
In case of voting after or at the end reminder will need to be sent may be once a week.
For posted paintings we will need to have some filters by color or by size, to let those that were posted first to be seen and be voted for. Or just sorting by day or by week of posting. May be to see them through scroll bar rather then going page by page. I think on average by a week around 200 paintings get posted. May be for competition it will be 400 per event?
Another thought - i hope you meant photos not real paintings sent to some physical address in some country? I guess one entry per user? I know you did not ask to ask more questions. 🙂 sorry.
I just mean I am alright with format of competition however I am not a competitive person. At all. I am sure people here will be interested.
Another format would be a Challenge rather than work for the prize. May be challenge fits better your idea of exploring one's own capabilities and hearing back from others and getting votes as success indicator. But for challenge I think time frames shall be shorter to make it tough and to step out of the comfort zone. May be 2 weeks only.