Help i dont know how to change the mojang.png when i load up the modpack i made

[Modernization]

So I made my own modpack- and when I load up the modpack it shows this forge loading screen with the little hammer (I attached the file). Can someone tell me how to change this so I can put my own logo there? I know that I have to make a file and name it mojang.png or something... but I don't know how big it should be ( what x what) and where I put it before archiving the modpack folder

[IJustGotNewSockz]

It's usually done by creating a texture pack.