I used the link that you attached to create an STL file of a butterfly image and I used the outer curve to shape the image. (Really easy to use and great page by the way). Download the file and it is a pretty decent file, with a size of 9.3x6.7 cm and a thickness of 2 mm. The only problem was that the model had some naked edges, but it could be solved with “meshmixer”. (If you do not have the software, you can download it for free or notify the printer center it could solve it).
About the printing. It is better if you print it vertically to obtain the best result. Printer without filling so light can pass through and play with the quantities of innerlines to obtain darker or clearer images.
I made a lamp once, in a different way, but so far is what I can share from my experience.