The user profile picture displays automatically when you create a list of blog posts using the Lively Blog Posts macro. If you don't see the user profile picture, you may have CSS in place which hides the user profile picture from that list.
On the actual Confluence blog post, the user profile picture does not display (only in the sidebar and in the comments section, which I understand is not what you want). You could write a user macro that displays the author of the page and modify the blog layout to include the macro on all blog posts. This thread should point you in the right direction - it's a user macro for the currently logged-in user but it should be fairly easy to change that to display the user profile picture of the author.
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