I've been following this thread because I've had the same issue. Tried this out, but the PNG file didn't work for me. So, I poked around Google and found
this. Turns out if you also have a slider theme, you need to modify that theme (posts #7 and #9), particularly if it's above any other themes in your Winterboard list. I have a slider them that kills the entire slider (bar, slider, text). Putting the PNG file in the slider theme solved my problem.
Just in case someone is running into any difficulty when running more than one theme affecting the lock screen.