I designed a locket using SolidWorks 2011. It is printed at Shapeways.
All the steps needed to design your own locket are shown. The steps show a basic form, not the actual locket I made. It's up to your creativity to make something beautiful.
As SolidWorks is not free, I imagine most people using other programs. I mostly used the features sketch, extrude and extruded cut. Its really basic, so there should be some free programs containing these features.
Step 1: Sketch
First of all, open your CAD program. Sketch a contour that wil be the most outer contour of your locket.
Sketch a second contour, this will be the outer contour of the lid of the locket.
Sketch a third contour between these lines. This is the maximum space your picture will get. This contour won't be visible from the outside. So even when you gave the other two contours a complex shape, you might want to keep this one simple. Otherwise, you will have to cut your picture in a funny shape. Also the contour should not be to close to the outer contour, as this should be at least the minimum wall thickness of the material you are designing for.
Step 2: Sketch Some More
The last part of the sketch is the ring on the top.
Draw two half cirkels with the minimal wall thickness between them on the top. By the way, my sketch is in mm.
Step 3: Extrude
We start with the back. Extrude alle the contours except the ring 2 mm (or again, you minimal wall thickness) towards the back.
Step 4: Extrude Some More
Use the same sketch for another extrude. Extrude everything except the middle part 3 mm (or 1 mm + minimal wall thickness) towards the front.
Step 5: Extruded Cut
Use the same sketch again. This time we will make a cut. Select the area between the centre en outer area. Extrude the cut towards the front for 1 mm.
Now you have the bottom part with space for a picture. If you want, you can round off the edges. Save the part.
Step 6: Second Part, the Lid
It's time to make the second part. We will use the same sketch. Make a new part and copy the sketch.
In a perfect world with perfect machines, the lid of the locket would be the same size as the cut in the bottem. However, machines are not that perfect. If you want the lid to fit, it should be designed a bit smaller. So scale your sketch with a factor of 0,95.
Step 7: Decoration
Draw something that will connect the part with the ring the lid. For easy opening of the locket, this decoration should stick out so it can be used for lifting the lid.
I made a little heart. The ring is connected to it, as well as the middle contour.
Step 8: Extrude Lid
Extrude the middle contour 1 mm towards the front and 1 mm towards the back. The size towards the back should be less as the size chosen in the second extrude of the bottem part (otherwise the lid will touch the picture and scratch it).
Step 9: Extrude Some More
Extrude the decoration and the ring 2 mm to the front. Save the part.
Step 10: Print!
The parts are complete. Save them as .stl files so you can upload them on sites like shapeways.com. Order them in the materials you like. Wait. Receive.
Combine the parts with a little metal ring or with a wire. Enjoy!