Hi Lars,
You can get tons of tiles on the Patreon page of PVGames, the tool will then display them automatically.
You can also use your own images with the „_Tile“ suffix(eg „Wood_Tile.png“) check this guide for more information: https://nightmare82.github.io/LevelEditor.html
About the object layer : by default it only displays the first 200 entries but you can change that by setting the „max objects“ in the options, you can also filter by directory.
let me know if you have any questions,
Martin