This paper reports a study assessing a new computer tool for cartoon storytelling, created by the authors for a target audience in the upper half of the English and Welsh Key Stage 2 (years 5 and 6, covering ages 9-11 years). The tool attempts to provide users with more opportunities for expressive visualisation than previous educational software; its design was motivated by earlier work connecting "moving image literacy" with print literacy, and it was used here in storywriting preparation work: users first visualised a known story, then wrote their versions of it based on the cartoons they had made. The stories produced are compared with stories written using two other preparation activities, one a pencil-and-paper cartooning exercise and the other a teacher's normal planning session, which also resulted in a retelling of a known story. The study finds that no one preparation process had a noticeably different effect on the final written stories; however, the cartoons produced with the software are found to be quite different to their paper counterparts, showing a greater variety of character action, pose and interaction, slightly more variety of camera shot distance, and more pictures. Children's and teachers' reactions to the software tool are also discussed.