All local files referenced by the src attribute of the <img> tag should exist.
With this small site structure:
site
| index.html
| page1.html
|
+---css
| style1.css
|
+---img
| pict1.png
|
\---js
script1.js
If index.html contains this content:
<body>
<!-- ... -->
<p>First image: <img src="img/pict1.png" />.</p>
<p>Second image: <img src="img/pict2.png" />.</p>
<!-- ... -->
</body>
The first <img> tag does not trigger any ERROR, because the file img/pict1.png exists.
The second <img> tag triggers an ERROR, because there is no file img/pict2.png.
Remote resources (for example starting with https://) are not checked by this rule.