Right click on the image of your layout and choose "copy image URL."
Paste this link into the image button on your forum post.
Don't forget to add the link too! In order to do this, while on your layout page highlight the link to your layout page at the top:
Back in your new forum comment highlight the image info you just inserted and click on the link button:
Click on the insert/edit link button and in the url box paste the link to your page and click OK:
Now your layout will show in the forum comment and be linked to your gallery page.