How to add custom field to image

I have searched, but haven’t found a clear explanation of how to accomplish the instruction to carry out the following:
Add images to posts as follows:

Edit the post to add the image to
Add a custom field called “slider”
Use the path to an image for the custom field value. This can be one in your media library or use an absolute path from another site if you have permission to do so.
That’s very important – image leeching is naughty.
Update your post.

Could someone explain in layman’s terms how to do this? I see nowhere to add a ‘custom field’ in my image editor in wordpress.
Thanks

http://wordpress.org/extend/plugins/scrollarama/