Print Render Image field in custom-node.tpl.php

great module. I'm now on media modules changed, but I forge and made ​​node.tpl.php.

Error Messages Notice: Undefined index: product:field_images in include() (line 18

Unfortunately, now the images no longer appear on <a href="<?php print $node_url;?>"><img src="<?php print render(image_style_url('product_medium', $content['product:field_images']['#items'][0]['uri'])); ?>" alt="product"></a>.

Can someone tell me how I can view the pictures again?

I'd appreciate any help. I do not like to put everything back now. Would like to continue to use media modules.

Best Regards