Hello Jon,
img-alt-require rule is from previous version of HTML Linter in Dreamweaver which is not applicable with the current version within Dreamweaver CC 2017.
I have logged a bug to replace img-alt-require with alt-require in HTML.htmlhintrc.
You can enable the warnings for missing alt attribute in img tags by adding below rule in HTML.htmlhintrc:
"alt-require": true
Regards
OK, so to fix the problem manually...
Go to Edit > Preferences
Choose Linting
Highlight HTML
Click Edit & Apply changes
Change the attribute "img-alt-require" to simply "alt-require" leaving the value as "true"
Save and close the HTML.htmlhintrc file
I can report that the above does highlight the line numbers with <img> tags that are missing alt attributes.