How can I attach images to a post which have already been attached to other posts without having to upload them twice?
I don’t want to insert the images into the post, I simply want to attach the same set of images to multiple posts.
As of WP.3.0.1 it does not appear that this is possible, however, if the “Media Library” tab of the “Insert Image” wizard had a column labeled “Attach” with a checkbox next to each image in the media library, this could be easily done.
How difficult a task would it be to add this functionality?
As you’ve noted it’s not possible out of the box. The feature was supposed to make it into 2.9, got postponed until 3.2, and assuming they reject the post2post table mike and I crave for it’ll probably be postponed again.
You’ll have better luck creating a custom taxonomy in the meanwhile, and “tagging” images tied to this or that post accordingly. That’ll give you an n-n relationship.
As for difficulty, I’d say not easy. Adding extra fields in the media upload/insert screens, in so far as I’ve experienced it (i.e. a huge media plugin) is a bloody mess.
Some of the better plugins aren’t available on the WP.org site, apparently because of SEO reasons, even though they allow other developers to stuff their plugin descriptions with affiliate links or keyword optimzed links for commercial terms.
Anyway here is a packaged solution.
http://shibashake.com/wordpress-theme/super-wordpress-gallery-plugin