I should have mentioned that I stained the carving white before I poured the epoxy on the image I posted. All the others I did were on bare wood.
All the engravings on the website were done on hard maple which I've found works well. It's very dense with a tight grain so I get good engraving resolution and it's light color is closer to white than most other species. I use Mixol (from Woodcrafters) to tint the epoxy. The picture I posted on the forum is on wenge which I had given to me. I prefer not to use exotic wood but it was very dense and it worked well as long as I stained the relief white before pouring the epoxy..
The bubbles can be tricky. A few tips are to mix the epoxy carefuly to reduce the amount of air mixed in. Warming up the epoxy and hardener thins it out helps the bubbles rise but they still seem to float to the surface and stay there. Carefully heating the epoxy with a torch will pop those bubbles that float to the surface. The epoxy I use has 3 choices for hardener, slow, medium and fast. I use the slow hardener to give the bubbles more time to rise but I'm not sure that makes any difference. If you can put up with the smell then polyester works similar to epoxy but doesn't bubble as much. You will have to sand the surface flat because it tends to dry with an orange peel texture and it can stink up the house pretty bad.