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...

So, you'd like to customize your Mac OS 7, 8 or 9 desktop by changing the Apple logo in the top/left corner? You know, you can customize way more than just this using one of the thousands of pre-made Kaleidoscope Schemes (look inside kaleidoscope_schemes.zip).  And of course, if you're a programmer, you could use ResEdit and fiddle with hex code and try to replace your System file's icon resource, but oh man, that's a risky...