This experimental online study investigates the influence of the depiction of ideal body images typical for Instagram on body dissatisfaction in young adults and also examines whether a self-discrepancy in the perception of one's own body can qualify the influence as a mediator variable. A quantitative mixed design was chosen. The subjects (N = 66, 72.73% female, age M = 24, SD = 2.75) were randomly assigned to the two an experimental group with idealized image material vs. a control group with average image material typical for Instagram. As part of a repeat measurement, the baseline level (pre-measurement) and both variables were measured again after exposure to the post-measurement for the criterion body dissatisfaction and for the mediator variable self-discrepancy. The mixed ANOVA to test the central hypothesis shows that the two study groups differ significantly in their mean values across the measurement times. Body-dissatisfaction increased after exposure to ideal body shapes, while it decreased in the control group with average body shapes. The discrepancy facet Actual/Ideal Self was included as a covariate in the mixed ANOVA; it also influences body dissatisfaction after exposure to the visual material. The determination of the directionality of the influence (GLM mediation model) was not-significant. The discussion opens up the debate as to what extent the self-discrepancy facet is the cause of body dissatisfaction (as a mediator variable) or rather part of body dissatisfaction itself. The limitations and the outlook provide approaches for future research.