Try this:
Log in to Image shack and the look for a button to the right almost at the top that says "My Images" and click on it. You should now see small thumbnails of all the images that you have uploaded. Find the image that you want to post on reef central and you'll see a bunch of small icons to the right of that particular thumbnail. Click on the top icon (should be a blue lower-case letter "i" in a white circle). When you click on that a window will pop up with options for sharing the image. In this window there's a box next to "Full Size"... make sure there's an X in that box. Just below that box there will be a bunch of code with "Forums:" written to the left of it. If you click on this code, it will highlight the entire code. Right click on this highlighted text and select copy. Then go to the page on reefcentral and right click in the text box for a quick reply. Select paste and it should paste the entire line of code into the text box. Click on the button that says "Post quick reply" and the image should appear full size.
For what it's worth, you may want to look into PhotoBucket instead. IMO, it's much easier to use and it's also free.