Pictures are often used in standardized educational large-scale assessment (LSA), but their impact on test parameters has received little attention up until now. Even less is known about pictures’ affective effects on students in testing (i.e., test-taking pleasure and motivation). However, such knowledge is crucial for a focused application of multiple representations in LSA. Therefore, this study investigated how adding representational pictures (RPs) to text-based item stems affects (1) item difficulty and (2) students’ test-taking pleasure. An experimental study with N = 305 schoolchildren was conducted, using 48 manipulated parallel science items (text-only vs. text-picture) in a rotated multimatrix design to realize within-subject measures. Students’ general cognitive abilities, reading abilities, and background variables were assessed to consider potential interactions between RPs’ effects and students’ performance. Students also rated their item-solving pleasure for each item. Results from item-response theory (IRT) model comparisons showed that RPs only reduced item difficulty when pictures visualized information mandatory for solving the task, while RPs substantially enhanced students’ test-taking pleasure even when they visualized optional context information. Overall, our findings suggest that RPs have a positive affective and cognitive influence on students' performance in LSA (i.e., multimedia effect in testing) and should be considered more frequently.