Research suggests that explanatory pictures support learning, whereas pictures that distract processing resources from the main ideas of a text may impair learning and are considered as seductive illustrations. However, non-explanatory pictures that are related to the text and that do not tempt readers to focus illustrations more than the text's main ideas might improve learning through spreading activation processes during learning. This effect might be reflected in memory measures or in metacognitive measures. Therefore, in two experiments, we tested related decorative pictures for positive effects on knowledge and transfer performance and metacognitive monitoring. Results indicate positive effects of related pictures on knowledge acquisition (Experiment 1) and metacognitive monitoring (Experiment 2). In neither experiment, related pictures enhanced transfer performance or interest (measured as triggered and maintained situational interest). This pattern of results can be explained by automatic mutual facilitation of related materials based on spreading activation processes.