These are stacking coasters with hidden internal lithophanes. The lithophane is inside the coaster, so it's hidden unless looking at it with a bright background light. There are three variations, each having a different pattern of ridges on the top. The ridges are meant to prevent a flat bottom glass from sticking to the coaster. The coasters are in the shape of an 11-sided reuleaux polygon (the shape of a Canadian Loonie coin) and the perimeter has a beveled edge so they stack nicely inside one another.
Included for each variation are two coasters with images, one solid coaster with no image, and the OpenSCAD file used to generate them so that you can make them with your own pictures. The picture should be in greyscale with high contrast to get the best effect and with some pixels completely black so that the layer on top of the picture has some places to print directly on top of. The lithophane is only 2 mm in height, so don't expect a lot of detail.
The coasters with lithophanes must be printed with 100% infill so that the lithophane prints correctly. You'll need to print with a light colored filament to allow the light to pass through.
0.2 mm first layer, 0.12 mm all others