How about i walk you through it:
When you open Imageshack, you'll see the Browse button. Once you click it, and select the image, click Upload button. A new window will open. You'll see that picture in a smaller version. Click on that picture, and it will open in a new window in the right size. In that new window where the image is, you'll find your URL up there. Copy that URL, and use it to pos the picture in Pokecommunity, by pressing add picture button when you're editing post, and paste the URL in that small new window. And you're done! :D