Hello! This unfortunately isn't documented well, since we mainly meant for assets exported from Pulp to then be imported into Pulp. But it works with any properly formatted .PNG!
So, your file should be a set of 8 × 8 px tiles. Save it as [name]-table-8-8.png. In Pulp, go to the Game tab, and down to the Assets section. Click Import next to Just tiles. If all has gone well, the new tiles should be available in the Room section, under the World layer. I hope that helps!