I saw that picture a pile of ballots on the ground. Not sure if it was real or contrived. Its hard to know if that's a common occurrence, or if it even happens at all. But most (surely all) states require a signature match among other things to verify the ballot. Letter carriers who do that should simply be fired.
If on the other hand those are general blank ballots, then it would be hard to commit voter fraud. The reason is that anyone trying to "harvest" those ballots would have to know the personal details of the person they were trying to impersonate.