This is a bug I mentioned in my post here. The variable $class
is not initalized.
To fix it, open bootstrap_shortcodes.php
in your editor and go to line 1734. Insert the line
$class = "";
so that the code looks like this:
$class = "";
$class .= ( $atts['type'] ) ? 'img-' . $atts['type'] . ' ' : '';
$class .= ( $atts['responsive'] == 'true' ) ? ' img-responsive' : '';
$class .= ( $atts['xclass'] ) ? ' ' . $atts['xclass'] : '';