I had the same problem when I started. They seemed to stay great with hot glue, then the next day were all popping off. I asked what everyone did & it worked great!
I purchased a Xyron sticker maker & the permanent adhesive & run my images through it to make a sticker. Make sure the bottle caps you are using do not have the rubber liner inside. If it does, like mine did, you can boil them for like 15 minutes & peel off each one with pointed edge tweezers. Pain in the butt!!
Once that is done, just stick the image with sticker on it to the cap & add your epoxy. If you use a magnet, I use a jewelry glue to attach it to the back. I will list everything I use. Make sure if you use this glue that they do not all stick together when they are dry, it always leaves a weird powdery film on my epoxy.
Hope that helps, they are very simple once you find all of the right products to use!!
Here is the sticker make: http://www.joann.com/joann/catalog/p...RODID=prd11364
Make sure you use the permanent adhesive!!
Here is the glue I use: (amazing!!!!!!) http://www.google.com/products/catal...ed=0CDIQ8wIwAg