Yes, as esmi said, DO make a child theme and re-install a clean copy of the default theme -- you will save yourself a lot of grief down the road!
I looked at your site and I think what you want to do it add the float:left to the following CSS (line 194):
.one-column #page {
float: left;
max-width: 690px;
}
It's still going to look kind of strange when the browser window is narrow -- as the background will disappear. To change that effect, you'd probably want to use a fixed width theme -- or adjust this one accordingly. That has it's own disadvantages, especially with so many smaller devices these days.
Putting the image in the body background creates a number of problems...as you can see. You might want to rethink how you are designing the site and the layout...