Might you be able to provide a screenshot showing the icon you wish to change?
You could always use a mix of the following:
Stealth activities which enables you to set up activities (with their original icons), hide and make them available to your participants
Labels / Text and media zone with your own images and the activity names
Activity name autolinking so the activity names in the Labels appear as links for your students
Stealth activities which enables you to set up activities (with their original icons), hide and make them available to your participants
Labels / Text and media zone with your own images and the activity names
Activity name autolinking so the activity names in the Labels appear as links for your students