What I did was put the images of my login page in the "portal-core" directory within jboss-portal.sar...
For example, I place image1.gif in portal-core/images and then modify the link to the image in login.jsp to src="/portal-core/images/image1.gif".
If you want to keep you images out of the portal sar then you can do the following:
In your WebContent directory of your war/web app (or the directory where your WEB_INF dir is located), make an 'images' directory and put all your images in there.
Then, in your jsp you can get to those images using code similar to this, assuming you have an image named myimage.jpg in the images directory:
<IMG SRC="<%= response.encodeURL(renderRequest.getContextPath() + "/images/") %>myimage.jpg">