Here is a Greasemonkey script for sending a flickr photo to many groups at once.
1. Open my blog post in Firefox
2. Download and install Greasemonkey
3. Click this link to Install the script.
4. Open your Flickr! Account and navigate to your Photo.
5. Click the usual “Send to group” button.
Here you can see a better GUI where you can do multi-select by using Ctrl Key+Mouse