This guide explains to you how to add your own custom spray in 3.1 version of Zombie Panic! Source.
1) Spray folder location
Go to steam -> steamapps -> common -> Zombie Panic Source -> zps -> materials -> vgui -> logos
As you can see each spray uses two files, VMT and VFT. It means simply dropping your pucture in this folder will not work.
2) Convert your spray
Use any spray converter which allows you to convert jpeg to VMT and VFT. For example you can use https://sprays_tk/ (replace _ with a dot). Just pick a picture you want to convert, choose pixel size (512 x 512 should be fine), click on convert and then you can download both files and move them into logos folder.
3) Equip your spray
Open ZPS, go to Custom content under Zombie Panic! Options and choose your imported spray. Make sure you do it before joining a server or otherwise you will not see the change.By broom_
