One of my favorite ways to decorate eggshells is with polymer clay, and many of the egg artists I admire who use blown eggshells for arts and crafts are also polyclay artists.
The first thing that I needed to know for my polyclay egg decorating was how to stick the polyclay decorations to the eggshell.
I figured that liquid polyclay would work, but it was hard to use, and somewhat expensive.
I also knew that E6000 would work on polyclay that had already been baked, because E6000 works on everything - and doesn't eat through the eggshell.
But I got the best results with simple white glue, on unbaked clay. I glued the decoration to the shell, and molded it to the curve of the eggshell, then backed the clay and the glue and the eggshell...
and voila! great results, secure, and beautiful.
This is one of my favorite uses for egg shells for crafts. Polyclay is an amazing art form, and when I combine it with egg art, I really have fun.