Lithophane - minus the background?

I used http://3dp.rocks/lithophane/ to create a lithophane. I want to extract/remove/delete the background. I originally uploaded an image into http://3dp.rocks/lithophane/ without a background but it creates one anyways. Any ideas on how to remove the "white space" background from an stl model?
Message me if you do because I haven't found an easy tool (software) to use that can do this. I think a cut-out look would make a really neat effect for lithophanes.