What do you have selected in AdminCP > Options / Setup > Options > Attachments > Default Image Processor?
This can occur if you use a custom processor (not GD or ImageMagick), AND if that processor does not extend from XenForo_Image_Abstract, AND if that processor does not function a thumbnail function. If this is the case, then it is a bug in the custom processor.
I have confirmed that all of the default image processors provided by XenForo provide the thumbnail function.
It is also possible if one of the following functions does not exist in your PHP installation:
imagecreatefromgif - PHP compiled --with-gd
imagecreatefrompng - PHP compiled --with-png-dir
imagecreatefromjpeg - PHP compiled --with-jpeg-dir
In this case, VaultWiki could handle the missing capability more gracefully. I've confirmed this as a bug for this reason.
In
vault/core/controller/dm/attach/thumb/xf.php, find:
Code:
$image->thumbnail($width, $height);
Before it, add:
Code:
if (empty($image)) return '';