First, open the photo in flickr you wish to add to a discussion, then click on "All Sizes" above the photo.
Choose the size you wish to post (I recommend either the medium or small size for the Themed Photo Projects.)
Scroll towards the bottom of the screen and you'll see "To link to this photo on other websites you can either:" and then there will be two different boxes with html code in them. You want what is in the first box where it says "Copy and paste this HTML into your webpage:"
Highlight everything in the text box and Copy, then go to where you wish to paste the photo into the discussion post and Paste.
You may want to hit Preview just to make sure it looks like you want it to! Then hit post and you are done!
To post a photo to the The Group Photo Pool;
First, go to the photo in your photo pool that you want to upload to the pool. Above your photo you will see a number of options depending on your settings.
Click 'Send to Group' button and a drop down list will appear listing all the groups you are a member of. Select the group you want to add it too and then confirm your choice in the pop-up window that appears.
And that's it!
NOTE: You must be a member of this group to add photo's to the photo pool. Remember that the groups guidelines for adding photos to the group photo pool states you can only add 5 photos per day.