Thank you for posting your openscad file for generating snowflakes. It was exactly what I needed to make my lithophane frame. I posted the stl file for my 2014 frame here:
More information about this project can be found on my blog:
Btw I solved your problem with combining the images.. I used Tinkercad to "merge" your snowflake STL and my own Lithophane ..
Very cool - I love what you've done with it!