You can change the images using the supplied psd files. If you download the reset.zip the psd files are in the source folder i.e gk_cherrydesign_rest_files_J!15source
You will need photoshop or an application which can open and edit psd files.
The actual images for each style in the template can be found here ../images/style1/btn.png this image the sprite used for the button/hover.
You could edit these directly with any image editor or just make your own new buttons using these as a template/guide.