You can remove PSD and XCF files. You can also delete background and foreground image files you do not need (they may be from custom menus that the author created for their own use or for systems you are not supporting in your own configuration). Same goes for logo files. To be honest, since I find myself continually tinkering and adjusting things, I usually keep the PSD and XCF files at least, and purge the custom or system backgrounds/foregrounds that I am not using or will not use.