For XenForo 2.2.x with article forums without a attachment set in the thread. You can define a default cover image for the preview style, not for thread view, so the are no blankets anymore in the preview grid style.
Installation
Auto-installation
Go to ACP->Add-Ons and upload the ZIP file
Manually installation
Upload all files in /upload folder in your forum root
Got to ACP->Add-Ons an install the new listed add-on
SettingsThe asset upload field is only available when add/edit a article forum with preview style.