Hi,
It's been a little while, but I'm pretty sure that this is all I did:
Go to Administration > Components > HikaShop > Configuration >
Then look for the second set of tabs below "Configuration" that begin with "Main, Checkout, Files...", and select "Display".
Scroll about halfway down the page, until you see the blue "CSS" heading on the left. Click on the pencil and paper link to the right of "Front-end CSS File" and copy/paste this at the top:
#hikashop_product_image_main{
text-align: center;
}
Hope that helps!!!
Cora