I finally disabled the “Upload” button from Image window dialog (from default workflow). However, the error message is too confusing.
This is what it said: An error occurred while pre-processing the request submitted by the host (-not available-). An exception occurred while processing the “Java/global/percussion/workflow/sys_wfAuthenticateUser” extension: com.percussion.security.PSAuthorizationException: User is not authorized to create this type of content.
So it should be “Sorry, you are not authorized to upload any images” blah blah something like that.