Ok, i fixed my problem, why some images cannot display properly or the image path missing "/forum/"
Here is the solution:
Edit the Images path in FULL URL such as "http://www.domain.com/forum/images/icons/icon1.gif" ...etc
There has 3 place to edit it
1. Style Manager -> Click GO to edit templates-> "Image Paths"
2. Post icons under Admin CP
3. Smilies under Admin CP
Thants all !!!
Ps: thanks for people who was helping me