<form action="changepicture.php" action="POST">
<p>
<p><strong>Новый аватар</strong></p>
<input type="file" name="picture">
</p>
<p>
<button type="submit" name="do_changepicture" class="btn">Изменить</button>
</p>
<a class="authlink" href="index.php">Главная</a>
</form>