XnView Software

It seems that removing this registry key solves the problem with associations (maybe i deleted more keys - i don't remember - if it doesn't work - report me). If you have XnView installed (or, maybe, other programs that create 'Open' menu element for images) then 'Preview' disappears from context ...

try to remove this registry key:
HKEY_CLASSES_ROOT\SystemFileAssociations\image\ShellEx\ContextMenuHandlers\ShellImagePreview