I had an idea, if you have above so many posts, the frequency that you need to complete these goes down? So after 10 posts you only have to complete it one of three times, after 20 one of four, after 30 one of five, etc. until at 100 its one of 50 times, 1000 one of 100 times, 10000 one of 1000 times, etc.
And then if some dedicated person writes something to do this anti-spam solving or just manually solves a bit in order to spam, then the mods could increase the number of times for that account.
I don't see why somebody who's an active positive contributor should need to do all these silly drag and drop image things.