I made a light box - two, actually - out of random, cheap elements. One is a small shipping box and the other was a printer box. They cost me nothing except the poster board and tissue paper to go on the sides, and they've served me pretty well.
Looking at a pair of your earrings, I'd say zoom in. You want to get detail shots. Do you have a camera with a macro setting? It's the little flower and allows you to get very close while still being able to focus. You don't have to aim for an overhead shot, either; sometimes angles work better, and as long as the buyer gets a sense of what it'll look like it should be fine.
I use GIMP, a free program, to lighten my pictures up. No matter how much light I have available, I have to mess with the levels to get rid of the shadows. Takes fives seconds, makes them look much better. I'd recommend it to anyone.