Practicing retrieval of vocabulary items from memory (e.g., with flashcard software or practice tests) is an effective study strategy to remember vocabulary over time. Retrieval practice is often implemented in digital learning environments that increasingly include multimedia (i.e., combining textual and pictorial information). However, it is unknown how multimedia design affects the benefits of retrieval. Therefore, the present study tested the effect of adding images during retrieval practice on students' learning, affective-motivational outcomes, and judgments of learning. We experimentally manipulated the presence and timing of images during retrieval practice of foreign vocabulary in three classroom experiments with students in secondary education. Across experiments, students' vocabulary recall on a posttest (1 to 4 days after practice) was weaker after practice with images that helped them retrieve the answer, compared with practice without images (Experiments 2 and 3) and compared with practice with images that appeared after the retrieval attempt (Experiments 1 and 3). Images enhanced feelings of competence but not enjoyment of practice. The majority of students recognized the negative effects of images on their learning only when the images clearly revealed the answer (Experiment 1) but--incorrectly--considered images that provided partial hints about the answer to be helpful (Experiments 2 and 3). Moreover, students consistently overestimated how much they learned with images that helped them retrieve the answer. During retrieval practice of vocabulary words, informative images are thus potentially harmful and students have limited insight into these effects.