Hi,
I suppose you are using VirtueMart 3.x & CleanMart v3.x
Open this CSS file:
templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexible.css
scroll to the bottom and enter this CSS line:
.FlexibleMainImage {
width: 90%; /* this is the max width of product picture, you can also set a fixed value, i.e: 600px */
margin: 0 auto; /* this is to center the picture */
}
regards.