Guess I misunderstood something.
Now, I deleted the files, reset the database to the original PP.
Downloaded the updated script and redone the import.
The files imported are the medium size, now the size is set as 600xsomething px. and not the original size.
You can see the results online. Let's stay on a wallpaper category. Where files were meant to be 1920x1080. Pantalla Ancha - Galerķa