During the process of creating my previous post, I discovered the thumbnail images were not rendering as expected for the custom images that I added to the out-of-the-box SharePoint /PublishingImages picture library (via a feature).

Here is the content of my original Elements.xml file (used to add the images to the library):

<?xml version="1.0" encoding="utf-8" ?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <Module
    Name="HomeSiteImages"
    Url="PublishingImages"
    Path="HomeSiteConfiguration\PublishingImages">
    <File Url="boat.jpg" Type="GhostableInLibrary"/>
    <File Url="fame.jpg" Type="GhostableInLibrary"/>
    <File Url="ropes.jpg" Type="GhostableInLibrary"/>
  </Module>
</Elements>

To get the thumbnails to render as expected, I simply modified the Elements.xml file as follows:

<?xml version="1.0" encoding="utf-8" ?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <Module
    Name="HomeSiteImages"
    Url="PublishingImages"
    Path="HomeSiteConfiguration\PublishingImages">
    <File Url="boat.jpg" Type="GhostableInLibrary">
      <Property Name="ContentType"
        Value="$Resources:cmscore,contenttype_image_name;" />
    </File>
    <File Url="fame.jpg" Type="GhostableInLibrary">
      <Property Name="ContentType"
        Value="$Resources:cmscore,contenttype_image_name;" />
    </File>
    <File Url="ropes.jpg" Type="GhostableInLibrary">
      <Property Name="ContentType"
        Value="$Resources:cmscore,contenttype_image_name;" />
    </File>
  </Module>
</Elements>

By simply adding the ContentType property (and setting it to "Image"), SharePoint automatically creates the corresponding thumbnail for each image when adding the item to the picture library.