This is what you need to do in flickr.
Go to your photostream.
Click on the photo you want to link to.
On the next page you should see 4 icons at the bottom right...one is for sharing.
Choose it.
Then choose BBCode and copy the link
Paste that link into your post here.
The image will then appear.
However, Flickr makes the link clickable etc, so you can adjust what you have pasted in and only use the IMG take piece and not worry about the URL tags.