I personally use SSH to get onto my phone. I followed this tutorial
http://talk.maemo.org/showthread.php?t=41422
then to modify the files
http://wiki.maemo.org/How_to_persona..._Unlock_Screen

There is other methods using the xterm, but I'm not yet familiar enough to suggest them, maybe someone else can post them.

Edit: Depending on if you installed any themes, it may not be under /usr/share/themes/alpha/backgrounds/ but possibly another folder similarly named to your chosen theme.

Also don't forget to backup