Alright people, Thanks a lot for your contributions !

So, i think , by reading you all, the very best is to base that on the blockchain block hash somehow like you said. This way, everyone can easily verify the random pick.  "Cloverme" solution is the best i can think of right now : Base the winner on the next block hash found :

Although, i have question. I am not good enough with bitcoin internal systems, so what i would like to know is :
What if i use the last ticket bought TXID instead of the next hash block.
Like that : i am user 10... i get a ticket, send the payment  : the website gets the payment, and the txid. And pick the winner with the last integer from the last txid ? Basically... i guess the question is: is there anyway for user 10 to know in advance what the txid will be ?

The reason i would prefer to base it on the txid, is that this way i have no lag : As soon as i get 10 tickets in, i can pick a winner, instead of waiting for the next block.

PS : Cloverme, gimme your btc address, you get the 0.1 btc. 
PS 2 : random.org is not free :-( If i wish to create a draw and keep the results online on their website, i have to pay for it, and the minimum price is 3.75 $. Actually, i think i am gonna make a website like random.org, payable in bitcoin, for a much lower fee th... That will be for another project !