Logon Studio is used to create and apply the logons. All it takes is a wallpaper to make a logon.

I don't know if the registry is changed, other than the registry entries that most programs put in when they're installed. If you mean does Logon Studio hack your system, no.

Very stable. No problems at all for 99% of users. Inevitably someone somewhere has problems with everything.. .

No risks.

The above should tell you my opinion of this software. I like it and I never turn it off! 

Okay then, thank you! The reason I was asking is because I was considering using it but then I read all these problem topics on the forum (black screens, nothing changing, error messages...) and became a little worried.

All did go well. It switched my background as promised, but the only problem is, I tried switching back to the default and it wouldn't lock the computer and display the new background as it normally would, thus leaving the background unchanged. Is this intended? (However I was smart and ripped the default background before switching and imported it and it worked, but I would still like to know why I am unable to switch back.)

Thanks!