Need image or infographic to span width of home page

I am designing an infographic to place on my homepage but before I got too far I tested it on the page. I want it to span the width of the homepage, minus the sidebars. But it won't span the page. I don't think it will go any larger than 600px and I think the width I need it to go to is 800px. I am a beginner. Please help. Site:

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