I've made thousands of tile charms and this is what I use....I use diamond glaze to adhere the image. Next I paint the back of the charm with 3-4 coats of acrylic paint (I like metallic silver). I'm not really careful with the paint process and am sure to get the edges and even the sides a bit so there's no resin sneaking into my image. Once it is dry, I use resin (envirotek). After that is dry (about 24 hours) I use an exacto knife to clean up the extra acrylic paint or any remaining diamond glaze that is on the glass. I use E3600 to attach the bail.
When I first started making these, I used diamond glaze on the back. I found that after a few wears, make-up or sweat could discolor the piece. My resin pieces are made to last and unlike with bottlecap images, if you have an air bubble or two, it's no big deal because it's the back of your charm. Hope that helps
I saw at a festival last fall someone selling these who had done nothing to the back of the charm, just paper.
I'm sure they didn't last long at all.