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I do some modifications, is this more clear or not ?
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Yes, thank you. Good job! 

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You mean a visitor reluctate to have a gravatar account but approve an avatar in snipsnap system? It seems to modify the database schema to save the avatar photo. But I hope we will not change any schema to preserve compatibility with official version.
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Well I was talking about any user. Let me try to explain better my question.
Let's say you have two users, X and Y, on a snipsnap with gravatar support. 

Both users want to have avatars on their comments and the admin of the blog has enabled the option in the Config Snip. The only difference between both users is that X as a gravatar account and Y doesn't have one. 

My question is, if Y wants to have an avatar, should we "make" him (or her) register to gravatar or, should we support some kind of local system - by local I mean within snipsnap application - that allows him (or her) to define an avatar?