Odd--I can't replicate this one on my phone. I used a 118x120 blank png (retina sized), renamed with the naming conventions above, and got exactly what I expected--no sign of a button, just the wallpaper of my theme, but when pushed in the usual spot, Camera+ popped open.
A difference between firmware versions maybe? If you follow the setup above with an actual solid image, what do you see? Path should be yourthemename/Folders/SpringBoard.app/CLButton.png (and
CLButton@2x.png, of course), with caps in all the right places, or it won't work. WB typically requires with 2x.pngs that both files be in the folder even if they're absolutely identical in size.
If all else fails, the same thing can always be done without WB by replacing/renaming the original files. Those are located in /System/Library/CoreServices/SpringBoard.app. I prefer to use WB to make later changes easier, but if it's the culprit here, changing the originals should work.