Currently, there is no way to do this in a vanilla WordPress install. Is there a way to retroactively attach an image from the Library to a post?
EDIT: Ah, let me clarify â when editing a post, and clicking the “Insert Media” button, I see no way to add past media. I can insert it into the post, but not add it to the post gallery.
This is not something that is currently supported in WordPress. There’s some discussion about it here: http://core.trac.wordpress.org/ticket/6820
One plugin which does enable this functionality is:
http://skyphe.org/code/wordpress/file-gallery/
This plugin does a few extra things, but one option it provides is that after clicking “upload/insert” and going to the media library tab, you can tick check boxes beside images and choose to attach them to the current post.
There’s also an old thread on WordPress support forums which pertains to adding a small bit of code to your functions.php file to enable this functionality but I think that only works within the Media Library section, not when in the Upload/Insert thickbox, so the above plugin might be your best bet.
http://wordpress.org/support/topic/detach-amp-re-attach-media-attachment-images-from-posts?replies=21
There’s a little glitch in Media panel, when attaching multiple files at once, do the following:
Right when accessing the panel, the Bulk Actions doen’t shows the Attach option.
Not sure why you believe there’s no way to attach an unattached item directly from the media library by default – in the list of media library items, You’ll see a column for “Attached to”. Items that are not attached to anything will say (Unattached) and then have a link to attach the item to a post.