I think the reason why Atheist are seen as untrustworthy is because theists tend to believe that morality cannot exist without their God telling them what to think. Of course, the truth is that morality can and does exist without a belief in God. Another problem is that a lot of non-theists are afraid to come out of the proverbial closet for many reasons.

The way that non-theists can change their image is by showing people what non-theists can do. That is why I encourage all non-theists to volunteer, help you community and share you beliefs! Let people know that you don't believe in God and that you are still a good person.