我有以下图像：

alex@alexvps:~$ sudo docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
<none>              <none>              70c0e19168cf        5 days ago          1.069 GB
<none>              <none>              c2ce80b62174        8 days ago          399.2 MB
<none>              <none>              60afe4036d97        8 days ago          325.1 MB

当我尝试删除其中之一时，我得到：

alex@alexvps:~$ sudo docker rmi 60afe4036d97
Error: Conflict, 60afe4036d97 wasn't deleted
2014/01/28 00:54:00 Error: failed to remove one or more images

如何删除它们？为什么会有这样的冲突？