"have been trying to put them into this post, but I'm clearly doing something wrong"
1.) In Flickr - you need to 1st make sure that all images to be shared are marked as "Public" - which yours are.
2.) Bottom Right of your Flickr Photo there is a small "right turn arrow" -click on that.
3.) Select "Embed" tab
4.) Select a "size" - I selected "Medium" here
5.) You will see a URL in the box - copy that and paste into your Net54 post
6.) post your message - Boom - Done