Steps to reproduce the issue

• Have a look at DocBlock description of HTMLHelper::image()
  https://github.com/joomla/joomla-cms/blob/staging/libraries/src/HTML/HTMLHelper.php#L566-L568

-1: Returns a <img> tag without looking for relative files

• Add PHP code in your template's index.php and see the output in HTML source code of loaded page.

$img = 'images/thing.jpg';
$name = 'Thing is Thang';
$attribs = '';
$img = JHtml::_('image', $img, $name, $attribs, false, -1);
echo $img;
exit;

Expected result

Method should return

<img src="images/thing.jpg" alt="Thing is Thang" />

because the last parameter ($returnPath) is -1 in code above

Actual result

Method returns only a path

images/thing.jpg

which should be only the case if $returnPath = 1 (and file exists in a calculated relative path; otherwise empty path).