Most online forums use BBCode markup, and Flickr has made it very easy for you to use the photos you have stored on their site in your forum posts. Since it's not immediately obvious how to do this, I've written a short tutorial.
First, navigate on the Flickr site to the photo/image that you want to include in your forum post:
Click on the “Share” icon — — which will open the sharing menu:
If it's not already selected, click on the thumbtack icon — — to select the BBCode markup. You'll also need to select which size image you want to use by clicking on the pulldown — — next to the current size. You'll now see all available size options:
Select the one you want to use and you'll find the BBCode now highlighted in the text area:
Using the normal copy function (PC: Ctrl-C; Mac: Cmd-C) copy this text and then paste into your forum entry:
And, after you've saved your forum post, this is what is displayed: