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Well I think the simplest implementation is going for a convention, for example, upload an avatar to the user's snip with name avatar.png or .jpg or any other image extension.
Then the user object would have hasAvatar() which would return true on and on the JSP we would do if snip.owner.avatar is true then show the image.
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Seems more easy, http://site.gravatar.com/site/implement says use a <img src="http://www.gravatar.com/avatar.php?gravatar_id=<%= MD5( email ) %>" alt=""> will show the image which saved at gravatar.

so, when show the comments, check snip.owner.avatar to decide write a code '<div class="gravatar"><img src="...."></div>' to his own comment or not.