Under Mac OS 9 and earlier, all the way back to the first Mac OS versions in the 1980's, you could display a custom boot picture in the background while Mac OS was loading.

To do this, you do NOT need to alter/edit any system files at all.  All you need to do is to save a copy of your desired startup picture into a specific format and place it in your System Folder.

Note: System 6 and earlier only supports black & white startup screens while System 7 and up support color startup screens.  Under earlier System versions, make sure that your startup picture is the correct size (your monitor's size).  On later Mac OS versions, it auto-resizes and auto-dithers if size and color depth are incorrect.

1) Download a copy of Graphic Converter if you do not already have this extremely useful image editor and converter.

2) Launch Graphics Converter and make sure that the Startup Screen export format is enabled.  Select in the menu bar: Edit > Preferences... then in the left pane window, click Save > File Formats, then make sure there is a checkmark next to "Startup Screen".  Then hit OK in the bottom/right corner of the window.

3) Open or create your desired startup picture and once you're ready, select in the menu bar: File > Save As... and then make sure that your file is named exactly: "StartupScreen" (no blank space) and that the format is set as "Startup Screen".  Then hit Save.  If you did not save it under System Folder, then manually put it there.

4) Reboot and see your startup screen displayed in the background while Mac OS is loading!