Basically whichever image you want, just right click, then go to properties and the direct URL should be there.
The easiest way to do it in imageshack (IMHO) is to:
1. Find the image you want
2. Click on the little (i) icon next to the image
3. Click on the thumbnail which has sizes underneath it (eg. 3264 x 2448 1932kb)
4. Image will open in a new window, right click, properties
5. Highlight the URL listed
6. Use the key combination Control+C
7. Then Control+V it into your post
(Sorry if the whole Copy/Paste thing is something which you already knew, just wanted to be thorough)
As newington said the Direct Link field will also work