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
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
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
Step 4: Extrude Some More
Step 5: Extruded Cut
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
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
I made a little heart. The ring is connected to it, as well as the middle contour.
Step 8: Extrude Lid
Step 9: Extrude Some More
Step 10: Print!
Combine the parts with a little metal ring or with a wire. Enjoy!