Sort images in image browser – how to?

Hi all,
because I want to have a gallery which is generated automatically by checking a post for image attachments I need to separate the post content and the images. So, these images are just uploaded to a post – thats it.
Now I want to have an optional possibility to sort these images manually. I was thinking about using custom fields. Maybe there is already an existing solution, does anybody knows one?
If not, how can I add custom fields to an image?

thanks in advance
Best