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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?
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I think, just have some description for user at registration page and let them know if he or she have a gravatar account, fill it on the email field can show their avatar in the snips.

Create a local avatar system seems a huge engineering.

However, maybe we can choose another way: use a macro like __\{gravatar:somebody@somewhere\}__ . It let user do what she or he like, but user must remember put the code in snip every time.