Quote from Silvio 1) It is possible to limit daily credit for an action? for example posting a picture will give you some credits but i do not want people start loading massive pictures to get credits (same for messages). I've seen that in some sites people delete and reload pictures to get infinite credits

No, there is no any limit. The fact is that the credits system was designed to monetize the site, i.e. to make money by means of selling the credits packs, but not granting the credits for actions like adding content. Moreover the limits will confuse the site members, because it would be not obvious if a user uploads 7 photos and receives only 5 credits. In any case it requires the code modification to add the actions limit.

Quote from Silvio 2) a payment gateway is needed or i could let it work it as it is? i'm not intended to sell credits. I just want people partecipate to get coins.

If you do not want to sell credits packs, there is no need in the payment gateway.

Quote from Silvio 3) it is possible to find a complete documentation of the product without just looking at posts to understand how it really works? :)

What product exactly are you talking about: Oxwall or User Credits plugin?
If you are talking about the User Credits plugin - you can find the manual here: http://www.oxwall.org/forum/topic/3178For the Oxwall documentation please go to: http://docs.oxwall.org/

Quote from Silvio 4) i need it for 2 website... there a special price for buying 2 licenses?

Yes, one licence per one site, please read our Terms of Use here.