From what I've seen locket pictures are faces only. You would need to take an image into PS, Use the Elliptical Marquee tool to cut out just the face and move it to a new image file.
Or simply print out a 5x7 image of the person you want to put in the locket and use scissors to cut out the face area. As to the actual size it would be hit or miss.
You can also use a Post Crop Vignette in the Effects area of LR, make the shape oval and the color white/bright with a hard edge. You would need to measure your locket and make a crop ratio of similar ratio to the width and height measurements.