These instructions presume you already have a Flickr account.
Sign into your Flickr account and go to the image you want to post on the forum.
In the bottom right hand corner, click on the little arrow icon.
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You will have this box which allows you to pick sizes and formats.
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Choose the size you want - I recommend the one shown (640x427). Then click on the URL link in the top box, copy this.
You can then paste this URL into your post on the forum.
Click 'Preview' to see how your post looks, or click 'Submit'to post it to the forum.
Also…
For those that like a cleaner look, you can trim away the bumpf that Flickr adds by deleting the sample text shown in red below:
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