If the images is a tad larger than the cap then that will leave to much room for prying off. This is the best punch to use: http://store.scrapbook.com/f-7436.html
Also, it will never be sealed perfectly to the cap unless you put resin over the side. Most people dont' do it that way. If you have a good punch, it will help prevent the peeling off. Oh and definately use Envirotex Lite. That will help TONS.