Make another copy of the link1 layer called link3 and place it directly beneath link2. Move it a little to the right of link2 so that the inner edges touch. Select the layer and CTRL+click to get a selection, contract the selection by one pixel. Select link2 and make sure you've clicked on the layer mask box rather than the image box and again paint black down the bottom of the link inside the selection. You should end up with a nice link chain like this: