Yes you are right you need to check width of sidebar. Then according to that you should put image in sidebar. I am using the same theme and default width is 220px under primary id in stylesheet.
i think changing these styles should do:
#container {
margin: 0 -330px 0 0;
}
#content {
margin: 0 340px 0 20px;
}
#primary {width:310px;}
depending on the way you added the image to the sidebar, you may need to adjust the number values.
if you are stuck, post a link to your site.
Here is what I did and this modification seems to work:
#container {
float: left;
margin: 0 -330px 0 0;
width: 100%;
}
#content {
margin: 0 370px 0 20px;
}
#primary,
#secondary {
float: right;
overflow: hidden;
width: 310px;
}
#secondary {
clear: right;
}