As an abundance of hardware and software tools is dramatically decreasing the cost and effort required to manipulate digital images, the risks and dangers associated with malicious attackers easily routing doctored images through computer and social networks to purposefully influence viewers' opinions, attitudes, and actions have never been more severe. While there is a growing awareness that images no longer represent an authentic proof of reality, there is a gap in research about the public's vulnerabilities to visual misinformation and how individuals make credible evaluations about image authenticity. Filling the gap in research, this work conducts a series of empirical studies to find out the social and cognitive heuristics online viewers use in image credibility evaluation and how such evaluations influence their attitudes and behaviors. The data from these studies will help predict the ways by which viewers are most likely to accept evidence that online images have been manipulated and how they subsequently revise their emotions and beliefs surrounding them. This work also looks at potential ways cyber attackers could use social media to subvert social order by spreading visual misinformation, and what strategies would be effective in combating such behavior. Results from this work will inform the design of software for forensic image analysis and will lay the grounds for new technologies that help Internet users in continuously assessing the veracity of the mediated visual hoaxes and scams they receive online.