Thanks for your quick response.
I know the problem now, 'cos I just import all my Joomla articles to K2, and that's the reason those images don't show up.
One more question:
I try to make my Homepage (mainpage) with Categories (index.php?option=com_k2&view=itemlist&layout=category&task=&id=), and select all the categories. But how and where do I control the images size?
Can I also control the images alignment? As fas as I know, I can see all the images are showing Top - Center Alignment.
Regards,
Joseph