I've been giving 8 another try and have been searching around for a way to change the welcome screen background, so far I have not come up with anything, is it not possible to put a nice background image on the welcome screen like on 7? all I keep finding is stuff about the "lock screen" and all that does is change the background while it's locked. it's still dark blue when you login again from locked. also can you remove your username from the welcome screen? and just have "welcome"? and it's really annoying that lots of sites have people calling the Start screen the welcome screen, which makes searching for windows 8 welcome screen info harder.
press "windows key" + c->settings->change pc settings->start screan tab. It will help you change start screen background (but not to some custom image)