Here is an app that will help you quickly visualize x and y image offsets. I think everyone has tried to eyeball offsets by tweaking the x and y a bit, hitting build again (and again) and then viewing your changes. Simply upload your image in the embedded app below and then adjust the sliders until the image is in the right place.
You can download a zip file (5kb) of the code here.
If you don’t see the slider bars in the app below you need to upgrade to a browser that supports HTML5 <input type=’range’> tag, or feel free to download and modify the code.