Have you checked if there is a file called "LockBackground.png" located in your theme? Use iphonebrowser or similar program to get access. If you have "LockBackground.png", just replace that .png with your own background. Just be sure you rename your picture to "LockBackground.png" (without the ") and unmark/mark the theme in Winterboard and respring.I've had the same problems with that theme and sublime lock HD since 4.2. I'm using slanted lock at the moment but it still won't let me use my own wallpaper.
I wish I could get my lockscreen looking like this again...
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