What startup pictures, and in which distribution?
If you are referring to the little penguins in the framebuffer console, they are built into the kernel, and can be located in the sourcecode. I'm not sure what is involved in replacing them, though surely it can be done somehow.
As for the full screen images, like for example when Ubuntu boots, they are often located in /boot somewhere, but the image itself could really be stored anywhere one chose to store it. It gets put on the screen by means of an 'initrd' or 'Initial Ramdisk' which is something of a preloader which loads the kernel and some basic other stuff of the initrd creators choice, before proceeding with the rest of the boot process.