Fix: Don't allow Image/Script HTMLProofer checks to fail

It turns out that I'd accidentally allowed this to fail, which isn't
what I want - I'm only happy with links being invalid.

Additionally, fix the failing `ImageCheck`.

Closes #762.

Rakefile

@@ -53,7 +53,8 @@ namespace :test do
         /meetup.com/,
         /www-api.jvt.me/,
         /haveibeenpwned.com/,
-      ]
+      ],
+      checks_to_ignore: %w(ImageCheck ScriptCheck),
     }
     HTMLProofer.check_directory('./public', options).run
   end
@@ -63,7 +64,6 @@ namespace :test do
     require_relative 'lib/checks'
     options = {
       disable_external: true,
-      checks_to_ignore: %w(ImageCheck ScriptCheck),
     }
     HTMLProofer.check_directory('./public', options).run
   end

content/posts/2019-10-20-indieweb-talk.md

@@ -247,7 +247,7 @@ For those of you that can't render this in your mind, it could render something
 
 <span>
   <a href="https://www.jvt.me">
-    <img style="height: 32px; width: 32px; margin: 0; display: inline-block" src="/img/profile.png" alt=""/>
+    <img style="height: 32px; width: 32px; margin: 0; display: inline-block" src="/img/profile.png" alt="Jamie Tanna's profile image"/>
     Jamie Tanna
   </a>
 </span>: