I've searched and searched for code (no plugins) to add custom fields in media library attachments. I finally found something that may work for those searching for it.
Check out this URL: http://www.icprojects.net/adding-custom-fields-to-attachments.html. So far, I have been able to see a custom field column on my media library and have added information on these fields. I have yet to test/try it out (i.e. displaying images on posts/pages based on the custom fields), but am working on a shortcode to make that work. Or maybe to find out that it doesn't work LOL
I'll post my findings soon. Good luck!