This is a patch we are using on our server for image_attach. It does two things: First, it allows you to attach existing image nodes instead of uploading new images. Second, it provides direct image_gallery integration if image_gallery is enabled. When uploading a new image, you have the option to choose which gallery to assign it to. And, when attaching and existing image, the selection is gallery-aware. Thumbnail previews are shown of all existing images.