HOW TO GUIDE:

I've done this on the 770 and the N810, so I have no reason to think it would be different on the N800.

Open xTerm, become root, and navigate to the following folder:
/usr/share/icons/hicolor/scalable/hildon/

The file names are:
qgn_indi_startup_nokia_logo.png (800x480)
qgn_indi_nokia_hands.jpg (800x480)
qgn_indi_progressbar.png (24x16)

Backup the originals (I just added "_backup" to the end of the filenames), then copy your new images over the originals and reboot.

If you want to change the progress bar, it may help to know this:
The image is 24 pixels wide. It will display the first 8 pixels, then the middle 8 pixels over and over (98 times total), then the last 8 pixels. It took me several tries to get this right, so I'll save you the effort of trial and error.

Hope this helps all who want to try this.