Sharing and viewing photos on social networking sites (SNSs) have been identified as particularly problematic for body image. Although correlational research to date has established that SNS use is associated with increased body dissatisfaction, only experimental studies can enhance confidence in the conclusions drawn. For this reason, this systematic review synthesizes data from 43 experimental studies (N = 8637; %F = 89.56; mean age = 21.58 ± 1.78) examining the effect of viewing idealized images (i.e., attractive, thin, and fit) and body positive content on SNSs on body image. Two studies were conducted on adolescents. Each study had slight variations in how the images were presented for each category (e.g., selfies and photos taken by others). The wide variability in experimental stimuli and psychological moderators used in the published research make a systematic review more feasible and meaningful than a meta-analysis. Findings indicate that viewing idealized images on SNSs lead to increased body dissatisfaction among young women and men. State appearance comparison (i.e., engaging in social comparison while viewing images) significantly mediated the effect, whereas trait appearance comparison (i.e., the relatively stable general tendency to engage in social comparison) was a significant moderator. Mixed results were found regarding the exposure to body positive images/captions. Viewing images on SNSs depicting unattainable beauty ideals leads young people to feel dissatisfied about their bodies, with appearance comparison processing playing an important role. More research is required to assess the long-term effects.