Ok, these are the things I've done relating to the topic title:
1. Uploading the larger version of images into /images/large
2. Renaming them with suffix _LRG (as what required in Zen Cart admin)
3. Ensuring the spelling of filenames were correct.
All of my products have no problems viewing in larger mode, except these two products. They just can't be set. Any reasons why?
BTW, cache has been cleared few times, but still same result.
Affected product link:
Please check. Thanks everyone!