Describe the bug
Image links are not verified by htmlproofer. Not sure if this is a bug, but it would be great if this plugin could prevent broken images. Right now I have to do a detour to find out about broken images ( mkdocs build, then run https://linkchecker.github.io/ on output)

To Reproduce

1. enable htmlproofer plugin in mkdocs.yml
2. add markdown file with non-existing image link to your mkdocs project (e.g. <img src="doesnotexist.png">
3. run mkdocs serve

**Expected **
htmlproofer complains about broken image

Actual
htmlproofer ignores image tags when checking for links

Environment (please complete the following information):

• Windows 10
• Python 3.10
• mkdocs 1.5.2
• mkdocs-htmlproofer-plugin 1.2.0

Contents of mkdocs.yml:

plugins: 
  - search
  - htmlproofer:
      enabled: !ENV [ENABLED_HTMLPROOFER, True]