So you have an appropriate image you want to use for your phone:
Perhaps something like this one?
To copy this onto your Magic you need to first install busybox.
Busybox is basically a small, but powerful shell that contains hundreds of standard linux commands.. like free, top, ipconfig, cp, rm, etc. You'll find it incredibly useful and invaluable once installed.
So if you have busybox installed you can install the splash screen above by first saving your image (like the one above) as boot.gif in your sdk tools directory (or any directory if you have that the android sdk tools in your path!) and use:
adb shell busybox cp system/media/boot.gif /sdcard
adb shell busybox rm -r system/media/boot.gif
adb push boot.gif /system/media/boot.gif
adb shell reboot
This will make a backup of your existing boot screen, remove the one off the system (or it's readonly) - and then push a new one out in it's place - and finally reboot the phone so you can see it!
Please note that this is not the first splash screen that comes up.. it's like the third or something.. the one that displays when it makes a sound.