Without widening the overall theme, you can achieve about 585px by adjusting the width of the content area and adjusting the left and right margins. This puts things a little close to the right sidebar, and to the title and navigation links on the left. Give this a try and see what you think.
margin: 0 0 0 1em;
In the maximum video/image width field down below the CSS edit area, change that width to 585 and save the changes.
You will have to delete and reinsert your images and make sure to select "full-sized" and they should come in at 585px wide, unless the original image you uploaded is narrower than that in which case it will come in at the width of the original image.