That looks like an empty data.img you've got there. Did ROM manager prompt you to pick what to backup? I don't recall it doing that. In any case, you have a data.img, but nothing to extract from it. It *should* allow you to still restore it, though, but you'd have no data. In the even that CW ignores the empty file, and leaves data alone, that could explain why it won't boot. It would use current data (if you didn't wipe). That doesn't make much sense , though. You said you restored this before? Because what I see from that screenshot is an empty data.img.
Anyway, go to the directory for the backup that *did* work (in /sdcard/nandroid_tmp) and let's see what'd in there.