With this method one could assign a new sky texture to every map in a pwad. Besides being an easier method to the one for original DOOM formats, one can use jpeg or png image file formats and the image can be a different size, although it is best to stay with a size which is power of 2.
For example this new sky image:
First insert the image into the pwad, in between TX_ markers, TX_START and TX_END:
As usual, the image must tile horizontally. Notice the size is 1024x256 instead of the regular size 256x128. Because the image is 1024 px wide it will span the full 360°.
Then insert a new lump MAPINFO to specify the sky's texture:
Because MAPINFO is a text lump, there is no compilation required.
Save the pwad.
What it looks like in-game:
Happy fragging.
Kappes Buur, Jan 18, 2020