Changing the width to 777px worked great for me and got rid of the horizontal scrollbar. This is the only way to get rid of just the horizontal scrollbar. And it is only 3 pixels off from 780px, so it shouldn’t screw anything up.
In main.html change the width of your image to this…
<img src="graphic.jpg" width="764" height="267">
764 is the maximum width of your image you can have without index.html having a horizontal scroller, but 777 is the maximum width you can have without main.html having the horizontal scrollbar.
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