Man I did expect shallow answers however, not this shallow - when I said "would THIS convince you" I didn't mean this specific photo... Jesus!
I meant would a high jump from an altitude similar to this convince you, AS, there would be no distorted lens there, it would be you on your own witnessing the Earth beneath you. Even with, say, how messed up your vision was, you would not be able to spot all the continents, apart from, obviously deducting the Earth was in fact round and a globe...
So, considering all the crazy replies I read on this site I think the cheapest way for an FE-er would be to do the high jump.
IMO it doesn't have to be as this one from the outer rim... Something a little bit less than 40kms would do as well.
(Apart from this you fail on the distance between argentina and australia, but lets not bring that up again since you are not unanimous about FE map...)