Zmieniasz po prostu miejsca kodu <img src
<div id="panel" style="float: right; width: 300px; margin-top: 15px;">
<form action="member.php" method="post">
<div>
<input class="textbox" type="text" name="username" value="Username" onfocus="if(this.value == 'Username') { this.value = ''; }" onblur="if(this.value=='') { this.value='Username'; }" />
<input class="textbox" type="password" name="password" value="password" onfocus="if(this.value == 'password') { this.value = ''; }" onblur="if(this.value=='') { this.value='password'; }" />
</div>
<div style="float: left; margin-top: 6px;">
<label title="{$lang->remember_me_desc}">
<input type="checkbox" name="remember" checked="checked" value="yes" style="vertical-align: middle; margin: -2px 3px 1px 0px;" /> {$lang->remember_me}
</label>
</div>
<div style="float: right">
<input type="submit" class="button" name="submit" value="{$lang->login}" />
<input type="hidden" name="action" value="do_login" />
<input type="hidden" name="url" value="{$redirect_url}" />
</div>
<div class="icon_button">
<a href="{$mybb->settings['bburl']}/member.php?action=register">
<img src="{$theme['imgdir']}/icons/lostpass.png" alt="{$lang->welcome_register}" title="Zarejestruj się" />
</a>
</div>
<div class="icon_button">
<a href="{$mybb->settings['bburl']}/member.php?action=lostpw">
<img src="{$theme['imgdir']}/icons/register.png" alt="{$lang->lost_password}" title="{$lang->lost_password}" />
</a>
</div>
</form>
</div>