The flickr site changed to a new version and it is not so easy to find how to embed pictures.
A couple of screen grabs are attached.
On Flickr look at the bottom of the screen and find the symbol that is like a square with an arrow coming out of it. ( circled with red pen on my picture)
Click the symbol and the sharing options should open, you need 'BB code' and a size of 'Medium 640' as larger pictures might cause problems with the forum page layout.
Copy and paste the code from the box and the result if it works correctly:
Just bouncing this up, instructions are still current although now when you click share it gives you the option of copying the BBCode so give it a go, its straight forward once you have a flickr account