So, first you're going to want to open up the minecraft folder,
I don't know about any other OS, but in Windows: Hold (WINDOWS KEY)+R, then type : %appdata%/.minecraft/bin.
Now that you have the minecraft bin folder open, find your "minecraft.jar", now right click on it, navigate to "7-zip", then click "open archive".
Now that your minecraft.jar file is open, open the folder named 'title', once you have that open, copy+paste 'mojang.png' to your desktop. Open 'mojang.png' in your favourite image editing program (I use paint.net) and modify it to your liking.
Now save it, and drag it into the 7-zip window of 'minecraft.jar' go back a directory (the up arrow in the upper-left corner) and delete 'META-INF'. Now close the archive and run minecraft :biggrin.gif:.