Link to a large image In D7 Commerce module, I've created a product_display content type.
With this content type, it is possible to create nodes from product entities that come from my store.
In these nodes, the images are shown but I can't figure out how to link them to a large image when hovered or clicked on.  

I guess this has to be done through a basic D7 configuration.
Assuming these are normal image fields, you can go to Structure > Content types, then select manage display for your product node type.

There, click on the settings button on the right side of your image field. A settings form should be expanded now, where you can configure in which image style you want them to show up and to what they should link. If you choose link to file, clicking on it will link directly to the original file (only).

An alternative is provided by the Colorbox module, which allows to display the images in a popup-like overlay. The same settings form should have more options to choose from once you installed that module.