D7: file_create_url() usable in node.tpl.php?
file_create_url takes a uri, such as public://science-of-rock_background.png

The revised:

$img_src = $content['field_image'];
$bgimage = file_create_url($img_src);

Would work if your field is only returning the URI.

And, for best practices, should set that up in preprocess_node, otherwise will work in node.tpl.php