Hi, I've been doing some tries with Eiffel Tower image doing these lithophanes. Results were fine so I'm sharing and now I can do it with my peoples' images :D
For first and second (flat), use 0.2 layer height or smaller so they get enough definition. Be carefull with stringing.
For third and fourth (curved), use what you want. Just don't use abs because of the warping xD.
I personally like positives, negatives only work with light. Details look better in both verticals (curved). Happy printing and post makes/comments please))
Better If you don't use ABS, specially if you are going to curved models.
Using light behind them makes better effect.
I used http://3dp.rocks/lithophane/ and an image from google, then played with parameters. Use images with high contrasts for better effect.