BACKUP FIRST BEFORE DO
it's easy! follow this :
1. name the new picture with weapon skins like:
2. make sure the format picture is .TGA
3. size must 256 X 64
after that, go to:
czero > gfx > vgui
put your new weapon picture.
choose copy and replace if you get a message.
now open CS:CZ and try.
P/S: Hope you understand this :P