The Offset effect isn't safe for negative values. The TiledBackground object is safe for negative values, but offsetting only works with power-of-two-images.
In your case the easiest solution is to use the tb object, because the image is of a power-of-two-size.
But for those curious: You need to do the wrapping on your own (until the offset effect is updated). Here is how (using the example file from above): [url:2whz849m]http://dl.dropbox.com/u/11182740/offset%20help.cap[/url:2whz849m]