Problem modding "specialty_nuke.iwi"

[deagle bear]

I tried to edit "specialty_nuke.iwi" which is in "iw_05", but I got a problem while saving the image. This file contains alpha channel and its size is the same in .dds and .iwi.
I tried to save using DXT1, DXT3 and DXT5 creating mipmaps and without create mipmaps
I saw something about save in "RGBA32" in a a post called " [Tutorial] full images list for hack "
If someone knows how to save the file, please explain me

(Sry, it's my first post and i don't speak english good)