In my case the answer of [Nathan Hase][1] helped with little modification. It turned out that item didn't have a reference to the image in custom image field. And because of that related image didn't published when you publish item. To fix this you need to register your custom field type:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <fieldTypes>
      <fieldType name="ImageEx" type="Sitecore.Data.Fields.ImageField,Sitecore.Kernel" patch:after="fieldType[@name='Image']"></fieldType>      
    </fieldTypes>
  </sitecore>
</configuration>

Where ImageEx is name of your field in /sitecore/system/Field types/Custom Field Types.

Also instead of new string Source you can use override string Source as this property is virtual [1]: https://sitecore.stackexchange.com/users/1150/nathan-hase

In my case the answer of Nathan Hase helped with little modification. It turned out that item didn't have a reference to the image in custom image field. And because of that related image didn't published when you publish item. To fix this you need to register your custom field type:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <fieldTypes>
      <fieldType name="ImageEx" type="Sitecore.Data.Fields.ImageField,Sitecore.Kernel" patch:after="fieldType[@name='Image']"></fieldType>      
    </fieldTypes>
  </sitecore>
</configuration>

Where ImageEx is name of your field in /sitecore/system/Field types/Custom Field Types.

Also instead of new string Source you can use override string Source as this property is virtual