FSX PMDG B737 Import Alpha to the Texture

I want paint a Livery for the PMDG B737. The Day Texture is already done, but i dont know what to do for the Night Texture. my english is not so well and i think i did a Mistake with translation. I understrand that i must copy and resize the Alpha to the pmdg_737NGX_fuselage_1.dds. I open the File with DXTBMP send it to Gimp. But the open File wont accept the Alpha. It´s equal what i do. Also if i paste it in pmdg_7378NGX_fuselage_master.psd - no Way to get a Night Texture. I not understand the Mistake.
Does anybody knows what i must do?
Thanks a lot.