That's theme specific; most themes are set up to display the avatar there, but some themes are designed with a more 'sleek' look in mind and leave that snippet of code out.
The bit of code is usually found in index.template.php and will look like this:
if (!empty($context['user']['avatar']))
echo '
<div id="db-avatar">'.$context['user']['avatar']['image'].'</div>';
Typically, that area of the main index page is called the "User Area". For Backstage's default theme, the user area looks like (pay attention to the //commented lines):
<div id="userarea">';
if (!empty($context['user']['avatar']))
echo '
<div id="db-avatar">'.$context['user']['avatar']['image'].'</div>';
if ($context['user']['is_logged'])
{
echo '
<ul>
<li><b>', $txt['hello_member'], ' ', $context['user']['name'],'</b></li>';
// Only tell them about their messages if they can read their messages!
if ($context['allow_pm'])
echo '
<li>' ,$txt[152], ' <a href="', $scripturl, '?action=pm">', $context['user']['messages'], ' ', $context['user']['messages'] != 1 ? $txt[153] : $txt[471], '</a>', $txt['newmessages4'], ' ', $context['user']['unread_messages'], ' ', $context['user']['unread_messages'] == 1 ? $txt['newmessages0'] : $txt['newmessages1'],'</li>';
// Is the forum in maintenance mode?
if ($context['in_maintenance'] && $context['user']['is_admin'])
echo '
<li><b>', $txt[616], '</b></li>';
// Are there any members waiting for approval?
if (!empty($context['unapproved_members']))
echo '
<li>', $context['unapproved_members'] == 1 ? $txt['approve_thereis'] : $txt['approve_thereare'], ' <a href="', $scripturl, '?action=viewmembers;sa=browse;type=approve">', $context['unapproved_members'] == 1 ? $txt['approve_member'] : $context['unapproved_members'] . ' ' . $txt['approve_members'], '</a> ', $txt['approve_members_waiting'], '</li>';
echo '
<li><a href="', $scripturl, '?action=unread">', $txt['unread_since_visit'], '</a></li>
<li><a href="', $scripturl, '?action=unreadreplies">', $txt['show_unread_replies'], '</a></li>';
echo '
</ul>';
}
// Otherwise they're a guest - this time ask them to either register or login - lazy bums...
else
{
echo '
<script language="JavaScript" type="text/javascript" src="', $settings['default_theme_url'], '/sha1.js"></script>
<form action="', $scripturl, '?action=login2" method="post" accept-charset="', $context['character_set'], '" style="margin: 0px 0;"', empty($context['disable_login_hashing']) ? ' onsubmit="hashLoginPassword(this, \'' . $context['session_id'] . '\');"' : '', '>
<b>Username: </b><br />
<input class="loginput" type="text" name="user" size="17" /><br/>
<b>Password: </b><br />
<input class="loginput" type="password" name="passwrd" size="17" />
<input class="logbutton" type="submit" value="', $txt[34], '" /><br /><td valign="middle" align="left">
<label for="cookieneverexp"><b>', $txt[508], ': </b>
<input type="checkbox" name="cookieneverexp" id="cookieneverexp" checked="checked" class="check" /></label>
</td>
<input type="hidden" name="hash_passwrd" value="" />
</form>';
}
echo '
</div>
Dunno how familiar you are with this stuff (I'm a novice myself, really), but in php (probably other languages too), beginning a line with // tells the browser to ignore that line. Any time you see a // it's a developer's comment for the information of anyone who looks at the code later to know wtf they're looking at, if needbe. Even though most of the User Area doesn't have anything to do with the avatar, I left it in just to illustrate the way it's arranged in the index.template.php file.
In this case, some of the sections of that code block are pretty self-explanatory:
if (!empty($context['user']['avatar'])) is pretty clearly saying if a user has an avatar, so by context we can infer that the next bit of code is for displaying the avatar.
if ($context['user']['is_logged']) obviously is checking to see if they're logged in and if so, the following code calls for the "Hello Nakatre", etc.
These SMF templates (most of them) are pretty modular in the way the code is arranged; you might even be able to just copy/paste the avatar code snippet above and insert it into your forum's index.template.php right before the 'logged in?' check. It goes without saying, though, any edit you make (no matter how small), backup that file first so if you break it you can just re-upload a working file and everything's okay again.