To gain root access, you have to install becomeroot AND type "sudo gainroot" at the prompt. Typing "sudo gainroot" won't work if you don't have becomeroot installed. For a list of Linux commands, http://www.linuxguide.it/linux_commands_line_en.htm is a good resource. As for a step by step, including exactly what to type, here goes...

Make sure your files are properly sized and named (see my previous post for this info). Place the files in the "Documents" folder on the device. You could use any folder, or the memory card, but for the purpose of this tutorial we need to be on the same page. Open xTerm and type "sudo gainroot". Type:

cd /usr/share/icons/hicolor/scalable/hildon/

This navigates to the folder where the startup screens are stored.

cp qgn_indi_nokia_hands.jpg qgn_indi_nokia_hands_backup.jpg
cp qgn_indi_startup_nokia_logo.png qgn_indi_startup_nokia_logo_backup.png
cp qgn_indi_progressbar.png qgn_indi_progressbar_backup.png

This will create backups of the files you are about to modify.

cd /home/user/MyDocs/.documents/

This navigates to the "Documents" folder, where you have placed the new files. This command in particular is case-sensitive.

cp qgn_indi_nokia_hands.jpg /usr/share/icons/hicolor/scalable/hildon/
cp qgn_indi_startup_nokia_logo.png /usr/share/icons/hicolor/scalable/hildon/
cp qgn_indi_progressbar.png /usr/share/icons/hicolor/scalable/hildon/

This copies the new files to the proper directory, overwriting the originals.

Exit xTerm and reboot. You may have to shutdown and reboot a second time before the new screens will show.