If you mean the actual product image, like Ismael said it would require editing the theme files which we wouldn't be able to direct you in.
However, if you are just looking to add an external product, you can change the product type to External/Affiliate product the the Product Type box when viewing your product editing page. This will then give you the option of putting in your own Product URL and to add your own text for the "buy" button.
The code above, is what you would put in the HTML tab of your product description to get an image inserted in with a link. You can also do this via visual editor by inserting an image and then applying a link to that image.