I cannot use preview function in Dolphin to preview JPG files that the size is bigger than 3MB. It is no problem when the photos’ size are smaller than 3MB. Do any one got the same problem?
Go to Dolphin settings and enable preview from general tab.
Go to the dolphin settings > general > previews
Look at the bottom of the settings box and there is a setting for preview size
It works. Thank you very much!! 
No problem friend