I didn't even try adding pics yet, but I think you can do it by just creating a .png file with the same name of the level and it should work.
Totally! I've played that version. I know what level you mean. But that clone it's a little creepy tough, there's hidden images in some folders... And the other Sad Satan ripoff that dude made it's like wth? Straight up n*zi shit... That dev creeps me out...
I know, I've tried that too long ago, but for some reason you can't have 2 exits or more in 1 level. Can't even do the most basic stuff with Terror Engine 😭