blupler, If you go to the setting menu of your player and select firmware version. If it is version 9.0.48 or 9.0.45 you will not be able to theme it as there are no tools yet that work with these versions.
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
Never try to load a pre-themed firmware to your player. Always customize your own firmware then there is no risk of killing your player with an incompatible firmware.