You have 2 options flash your player with firmware that has already been modified, a bit risky as you must be certain that the firmware is compatible with your player.

Secondly you can do a dump of your own firmware by using a program called S1fwx which will extract your players firmware to a file called dump.bin . Then open the dump file with a program called S1res, here you can find all the images on your player undr the "icon" headings then you just import your new images, save the file and flash back to your player using MP3 Upgrade tooll. When you replace images they must be in bitmap format and the same size as original images.

All these applications can be downloaded from the tools section in downloads.