According to iL, only smk files work with wc2 pvpgn:
PCX - has a palette problem
SMK - Starcraft, Warcraft 2, Diablo 1-2
MNG - Warcraft 3 only
So there's the reason for your color palette issue. However, someone contacted me last year with the same problem, he originally made his graphics in gimp, exported to gif, then converted to pcx with Irfanview. His solution was:
For some reason the [png converted to pcx in photoshop] you uploaded didnt work. I have no clue why. I opened my png banner with photoshop and converted it to the color palette from original warcraft2 bnet background graphics. And that worked. Even better [than] GIMP / IrfanView.
So he's saying a png worked for him? I know for sure this was on a wc2 pvpgn.