I can't help much (because I use the XP version) other than suggest you try other logons, and make sure that they are for Vista or Windows 7, and not XP, because they are not compatible with each other. Your problem could be as simple as having tried to use an XP logon on Vista. Welcome to the world of skinning--if you haven't already done so, check out the other progams as well.

Thank you for all your kind suggestions! I've discovered it was the packages themselves that were the problem. I'm not sure what was wrong with them-- the artist said they were for Vista/7, and they showed up the same as the sample logon I downloaded, but they still don't work. The sample one did, though, so I know there's nothing wrong with my computer or program. Again, thanks a lot for your help! 

Cavan1: I only found out about LogonStudio through deviantArt, actually, browsing their Vista customization deviations, so I know they have some. If you're interested, the package I downloaded was here: http://stamga.deviantart.com/art/Fresh-Logon-Pack-163422111 

Yeah, their tags can be a little misleading at times. I'm glad if I helped a little in pointing them out.