I would think that the ideal way to shoot stained glass would be with three light sources. The main light source should be behind the stained glass so that the camera sees light coming through the glass. Preferably, this should be an even light source. In the photo above, the trees block some of the light on the glass causing distracting "shapes" in the glass.
The other two lights should illuminate the front of the glass. These should be less bright because you want most of the light coming through the glass. These lights should be just bright enough to show the solder. They should be positioned well to the side of the camera so that their direct reflections don't hit the camera and show up as glare. The reason that you want two instead of one is that one would cause distracting shadows wherever the stained glass or solder isn't perfectly smooth. If you don't want to use two lights, you could just use a reflector for the other side.
This may sound complicated, but it really shouldn't be. Hand the glass in a window where you can look through the glass and see nothing but sky. Put a lamp a bit in front of and to the right side of the glass. Put another lamp in the same spot on the left (or have someone hold a something white on the left side just outside of the view of the lens. Take several shots trying with the lamps closer or further away to adjust the proportion of light from the lamps vs the light coming through the glass.
