Back in September of last year, I wrote a comprehensive tutorial for making marble magnets, an easy project in which images are placed under little glass bubbles in a way that really makes them come alive. In that post, I mentioned that the image pebbles could also be used in other projects, like concrete mosaics.
Kathy Cano-Murillo, one of my favorite craft gurus, recently posted the kind of mosaic project I was talking about: Concrete Coasters. The process for making them is similar to making a mosaic tabletop or paving stone, on a smaller scale.
After creating the image marbles, you mix concrete and pour it into coaster molds, then embed the marbles in the molded material and allow it to dry for at least a day. After removing the concrete coaster from its mold, you varnish it to seal it, and add felt padding to the bottom; if you like, you can further decorate the coasters until you end up with something like Kathy's results.
The one thing I would do differently than Kathy? I don't think white glue (also called PVA glue) is the best thing for gluing images to marbles; glass isn't porous, and that kind of glue is most appropriate for porous materials. I recommend using E-6000 glue instead. However, the fact that you're sealing the marbles in concrete probably makes it a moot point.