Why can't I add a group of images from the Media Library to a gallery?
Hi,
You would need to use the Add Media on the Add New — or Edit — Post/Page screen, then upload the images that you'd like to use for the gallery on that particular post or page.
If you've already uploaded some images to your Media Library, they'll say (Unattached) in the Attached to column. You can check multiple images, then click Attach to attach those images to a post or page.
You can see the Unattached Files support page for some information and screenshots on reattaching files:
http://en.support.wordpress.com/media/unattached-files/#reattaching-files
a button could be added in each image's Show area or a check box could be added on the Media Library listing page to indicate which images are destined to be included in a gallery container.
I completely understand, and we're aware the Media Library can use some love. It's on our radar, but I can't give a time frame as to when that'll happen. We appreciate your feedback.
In the meantime, the above instructions will get you to attach unattached images to a post or page.