i think you need a better sprite editor
https://openxcom.org/forum/index.php/topic,7781.0.html
irfanview is not a sprite editor
it's just an image processor and works VERY well for openxcom purposes
It looks like irfanview 'optimized' the palette
nope, it didn't, the palette used is not optimized
(hint: don't use an editor that figures it knows what it's doing with the palette ... it doesn't, this is XCOM, we use our own palettes here. )
that's EXACTLY why I use irfanview for years now, almost exclusively (besides Photoshop)
it doesn't "figure it knows", it does ONLY what you tell it to do
2nd, the spritesheet didn't preserve transparency for palette id #0 -- and we like our 0 ids to be transparent
that's also not an issue with irfanview, it was the user's decision... as said above irfanview doesn't "figure it knows", it asks you for input