I was having the same problem (Win7 64bit). Stardock Support had me try numerous things, but nothing worked. After more than 2 weeks of trying one thing after another, SD Support gave up.
Eventually, I found the default background hidden away in System32\oobe\info\backgrounds. Changed the name to backgroundDefault.jpg.bak, and LogonStudio starting working.
I just made an account to tell you a big THANK YOU, because this's my first time using W7 and, hell, I started to panick when Logon didn't work