事故的照片惨不忍睹
The picture which was about the accident was terrible.
这是个从句,拆开应该是这样的：
The picture was terrible.照片惨不忍睹.
The picture was about the accident.照片是关于（那场）事故的.
你可能是不明白which后面为什么有个was,或者这个句子里为什么有两个was,拆成两个句子就明白了.