Just thought I would update this (as Flickr keeps changing their page layout).

When viewing the image you wish to share follow these steps:

1 - click on the "share" icon (the arrow) located at bottom right of page. The white box will pop up.
2 - then select BBCode icon
3 - choose the preferred size. I generally use large (1024 x 683) for horizontal images and medium (534 x 800) for vertical images.
4 - select & copy the code.
5 - Post the code directly in the body of the forum post.

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