I just went through the wiki and saw that Sakya advices not to mess with the default configuration! He wants to keep the default lockscreen functioning right if all else fails probably.

So what I would do is copy the default folder and .ini and rename them to, say, mylockscreen.

Paste them in /opt/qtlockscreen

edit the .ini and replace all "default" to "mylockscreen" in the paths

then change the background image to the one you like

that should do it