You should be able to click on a button that's over your picture on Flickr that says "All Sizes" if you click on the medium size and then look below the photo, you will see two boxes, one has much more information in it. If you copy and paste the text that's in that box into your post, it should paste the picture at the medium size (instead of uploading it to the forums). We'll get your posts with larger pictures yet
Watch in a few minutes for a graphical tutorial on posting images.