OK, in User Settings, I set the default picture path to be "pictures"
Whenever a user uploads a picture, their picture is stored there in 'sites/default/files/pictures'
But when we use Imagecache Profile and we view that user's profile, the profile page looks for the picture in "sites/default/files/imagecache/user_image_default/pictures/". Since the picture is not stored there, hence the picture is not displayed.
How to fix this problem??