In looking through the forums and searching Google for the answer to this, it’s amazing the answers you see out there. For a simple change like changing the main product image in Magento, we see answers that say it can’t be done, you need a commercial plugin or extension, you have to do some complicated scripting on your server, etc….
The real answer is very simple. Navigate on your server to /app/design/frontend/default/yourtheme/catalog/product/view/media.phtml
Find the line that is something like this:
$_img = ‘<img src=”‘.$this->helper(’catalog/image’)->init($_product, ‘image’)->resize(265).’” alt=”‘.$this->htmlEscape($_product->getImageLabel()).’”/>’;
and change that magic little number of 265 (or whatever yours reads) to the number you want.
Then, go to your Magento admin and clear your images cache. I always clear the main cache of Magento too, just in case. Seems like sometimes Magento recognizes file changes, sometimes not, so it’s best to always clear the cache in the Magento admin.
Go back to your product page and you should see your main product image size changed in your Magento commerce site.