itch.io

select the entity and press
Ctrl + U
or enter the console (~) and use the command  U  (or UNIQUE)

it's a bit convoluted but you can:
you need to put it in a .png file in   the    Zortch\editor\mapskin\   folder
and then add a new entry for the   Zortch\editor\mapskin\tiledata.script
file so to the editor can see:
e.g. ADDTILE   yourfilename.png  3560  1.0  

and then for the game to see it you need to create a   Zortch\data\mapskin\   folder
and put it in there too (don't have to edit scripts in this case)

the skybox is even more complicated:  
you need to create a    Zortch\data\sky\    folder
and inside that a folder for your skybox e.g.: Zortch\data\sky\yourskyboxname
in there you need six files:

sky_up.png
sky_rt.png
sky_lt.png
sky_ft.png
sky_dn.png
sky_bk.png

you can use for reference the existing ones: the  data.dat  is just a giant zip  file

and to make it work on the level you will need a    Zortch\custom\anynameyoulike.script
file and the command need to look like this:

ADDMAP yourmapname
SETMAPSKY  yourmapname      "yourskyboxname" 

to add insult to injury it will only work if you load your map from the custom map option:
for it to work with the editor PLAY command:
edit   Zortch\data.dat\data\map\maplist.script
and replace  the last two lines  ( SETMAPSKY  preview      "canyon"  plane  )

I guess I should start on a patch to make this easier 🤔