I've been making a bunch of tiles in external apps (for greater ease drawing stuff that's bigger than 8x8). I was able to import stuff fine into the World layer by naming the file NAME-table-8-8.png, but I haven't been able to figure out the trick to getting the tile into the Sprites layer instead of the world layer.
I tried pulp-tiles-layer-Sprites-NAME-table-8-8.png but this still imports to the world layer.