The problem with changing firmware is that it may not support the components that your player has, board type, LCD, button configuration, boot sequence, recording components, FM chip, speaker etc .... You do not run into these problems using your own firmware.
You need to first extract your firmware from your player using s1fwx.
Download
HERE
Guide
HERE
Then you can change the images using s1res
Download
HERE
Guide
HERE
Then reload your firmware to your player using MP3 Player Utilities Update Tool
Download
HERE
Guide
HERE
And you will find all sorts of theming resources
HERE