ALT for custom image

Hello. I have the following situation:
I insert a picture in the Media Library, also completing the “title” field requested. I do NOT need to insert the picture into the post, but i insert it as a custom field instead.

I read the image using
$values = get_post_custom_values("custom-picture")
then I display it using
<img src="<?php echo $values[0]; ?>" />

What is the code for getting the correct ALT (title) for the custom image? 10x.