Magento Category Thumbnail missing on 1.9

This issue occurs on fresh Magento 1.9.1 installations only.

The problem is that thumbnail attribute is created by Mage_XmlConnect module, which became inactive since 1.9.1.

So you can either fix this by enabling the Mage_XmlConnect module:

Open /app/etc/modules/Mage_XmlConnect.xml
Find the <active>false</active> line and replace it with <active>true</active>
Navigate to Cache Management page and flush cache storage.