Hi Nicholas,
Thank you for your advice, I tried changing the product page layout to "Tabular" and reverse as well,
however the issue still persists. and I also checked that if my customization really had affected by
removing all of what I did (Adding custom css codes and modifying in the show_default php), but it didn't help too.
I found out that though the main image has moved, changing values of characteristic (For us it's set as "Color")
in the drop down menu will recover the position of main image again, and once it has done, the position won't move again even
hovering cursor on the thumbnail area... it's strange.
By the way we are using a template called Helix, still not sure what the matter is.
Anything can we do? sorry the link can't be shared as this project can't be published,
but if we can send you a message to you privately, then it's possible.
Thanks!