I've been making some pendants and bracelet charms today. I'm veering more towards using resin at the moment as I love the finish it gives. I've tried loads of different varnishes and glazes to coat my picture charms but none of them have the shiny hard finish that you get with resin. I thought that Diamond Glaze, which is really popular would be the answer at first but I've found that although it gives a lovely clear shiny finish it can be marked really easily.
Using resin is quite a time consuming process though. Your pictures have to be really well sealed, particularly the edges, otherwise the resin will seep into the paper causing a wet looking stain. Once you've got your resin in the frames you have to babysit for a good 20 minutes I've found to make sure there are no air bubbles coming to the surface. These need to be dealt with asap. I use a little butane torch and gently wave it over the resin - this normally gets rid of them.
Some of the charms I use are a bit of a pain to fill with resin like these ones in this bracelet I've just listed on Etsy. I've found that a little bit of clear acetate works well to protect the picture and looks nice too.