you are seeing the page background image no offset because you have a bigger image in the header.
If you use the 'adsense.txt' style as a basis, there is a background image (see ato->Body, Text & Links->Body Style) that is setup to match the location of the page menu bar. If you use a larger image than is specified, you will change the height of the header area pushing the pagemenubar down wo it will no longer match the background image.
So if you do change the settings, you will need to eithor remove teh background image, or make a new one that matches where your menubars end up.
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Juggledad | Forum Moderator/Support