I use a glaze called Triple Thick on top of the images. It works great, but my craft stores locally just stopped selling it, so I need another option.
I also just use a dab of tacky glue to adhere my images onto the cap. I do like the idea of using the double sided squares from the scrapbooking section. That would work well too!
Another tip? When I first started with bottlecaps I purchased them from another crafter- she put a touch of glitter on top of the image/glaze. It just adds a nice touch. So I do that too!
Good luck- www.simplystitching.etsy.com