We describe the Image Tagger system - a web-based tool for supporting collaborative image indexing by students. The tool has been used in three successive graduate-level classes on content representation. To fully satisfy the class' requirements and provide support for student indexing activities, it was designed and developed iteratively in accordance with the feedback and suggestions from the students as well as the instructor. The tool was well received by most students. They expressed a positive opinion toward collaboration support and thought it enhanced the overall learning experience in the class' image indexing project.