1) On flickr go to the lower right corner of an uploaded picture and you'll see an icon which looks like an arrow pointing down at a flat line - click it and it will come up with a downloading series of options. Pick "View all sizes"
(the other links start a download of the photo at the various sizes
2) On the new page click the size you want to show from the auto generated selection.
3) Once you've picked the size right click on the photo and get its URL. This might be "copy image address" or "Copy URL" or "Image URL" or anything similar.
4) come to the forums and put [ img ] paste the URL here [ /img ]
Without any spaces in the [] and it should embed into the forum