If one has a more or less transparent subject, one can quite easily make a photo of it to look much like an X-ray image.

1. Desaturate the image
2. Invert it
3. Adjust white and black points and curves to suit
4. Selectively darken the edges here and there with gradient masks

Since X-rays pass through solid objects, because this technique uses visible light, it only works on subjects that allow light to pass through them, like the first image below, which following the above instructions produced the second image.