You mean ResEdit.

If you put a .pict file named StartupScreen, it'll place a picture there during startup, but you won't see the progress bar, extension icons, etc.  You can also place StartupMovie in there with a QuickTime movie, which it'll play before it begins loading the OS.

To change the picture of the actual startup screen, you'd need to open up the System Folder with ResEdit and then paste in your own picture.  Make sure you make a copy of your System Folder before you do this.