HTML in PHP problem [closed]

You get the error because you try to call php inside php. Try to use this snippet and see what is different:

<?php
if(get_field('post_image'))
{
    echo '<img src="'. get_field('post_image') .'" />';
}
else {
    echo '<img src="' . get_bloginfo('template_url') . '/images/images/default.jpg" />';
}
?>

Also pay attention that bloginfo immediately prints value, when get_bloginfo returns it.