I think I'm too stupid for that.

I have loaded the file you linked --ambientCG_256.sh3t and copied it, as you said.

it is in the furniture folder. Now I go to the furniture file at the top, then down to import furniture library. In the field that opens I click on furniture and then on the .sh3f file. Now I get a message saying that the furniture library exists. Do you want to overwrite this? Replace or not replace. No, I don't want either.

If I click on furniture, only an image appears.

If I now double-click on a room, change rooms, select floor on the left and texture at the bottom, only the few predefined ones are included. If I now click on import on the right and select image, nothing works either.

I have now found a carpet in jpeg format, copied it into eteks and placed it on the floor as a writable box.

Thank you very much for your effort.