Pmp inc, Depending on what chipset your player uses. If it is a standard Action chipset then:
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