From your main flickr page, click on the picture you want to add to the Labradors group. Once you're in that picture's page, there are menu options above the picture such as "rotate", "add note", "more sizes" etc. There is also one that says "Send to Group". Click on that, and it will give you a list of all the groups you belong to. Select the Labradors group, and your picture will be posted in the pool!