Custom field in image attachment page

Hi!

I have a blog and twenty twelve theme installed. I have some pics galleries and the image files are linked to attachment page (image.php). Everything is great and working fine.

Now, the thing is that i’d like to place custom links on the image attachment page (for example: credits, photographed by, etc.).

I’m not very good in coding and stuff but I’ve found out the wordpress custom fields might do the trick. I’ve added custom field to a post and using this code link
<?php echo get_post_meta($post->ID, 'go_to_sponsor', true); ?>
I get the custom field to posts and so on but when putting it into the image.php or attachment.php nothing happens… :(.

Anyone could point me on how to insert this custom field into the image attachment page?

Thanks!