To change an appointment/task type's icon, you can follow the following steps:
Step 1: Run MultiCalendar Client/Server Edition Client, then click "File"->"Custom"->"Event Type" , and a new window will open. In this window, you will be able to view the appointment/task type list.
Step 2: Select the appointment/task type you want to edit, then click the "Edit" button. The editor window will open and you can change the appointment/task type icon in this window. After that, click the "OK" button in the editor window to save it.