The first way is, once you've posted an image and are viewing it, just click on the Add Image link and a new submission form appears. It is most filled in already so you don't have to do much except post the image and write another comment if desired. When you view the image you post there you'll notice your first image appears as a clickable thumbnail. You can continue to add images in this fashion and the number of clickable thumbnails on each image will grow each time you do.

The way to link separate images that are already posted is to click on Tag under each image you want to link. They will appear as thumbnails in the upper left corner of the bugguide window. Once you have thumbnails there of all the images you want to link, simply click in the Link Images link under these thumbnails, and they will appear just as if you had joined them with the first method.