Changing wallpapers for the login/alt-ctrl-delete menus:
This one's simple. Download this. The rest is self-explanatory. However if you really need explicit directions, click the browse button, find the wallpaper, select it, you're done.
I know of a way to change the start orb on the boot splash, if that's what you're referring to. However, it could easily corrupt the boot manager. So far I've tried it several times, and it has not worked for me yet, although it has worked for other people. If you want to give it a shot, try this link. In case you decide to try, and corrupt the boot record, it's easy to fix. The program creates a backup of the boot files. Boot with a LiveCD, I used Linux Mint but anything will do, really. Remove the modified files, and rename the backup files to their original names.