Moving the sidebar to the bottom, and have it look right, would be very difficult, and would actually require access to the underlying theme PHP script files, and a good knowledge of PHP. We don't have access to those.
Posting a 4288 wide image full-sized on a blog is a problem on many levels. First off, who has a monitor that is 4288 pixels wide? Mine is 2560 wide, but I'm in a tiny minority, and even with my 30" display I would see only about half the width of the image and would have to scroll around to see the rest. I've run into a few sites that have images posted larger than my monitor, and in the first place, it takes way too long for them to load, and in the second place, you cannot really get a feel for an image if you have to see it in bits by scrolling about. You also have to realize that monitors are only 72, 96 or 120 dpi so uploading anything higher than that for viewing on the web is a waste of bandwidth. It also requires the browser to interporate the image to be able to display it at the actual dpi of the monitor. Web standard is 72 dpi and works best with all browsers.
If you upload a 4288 wide image at flickr, and then insert it into a blog at a smaller width (say, 960), there is no resampling that happens to retain sharpness, and it is left up to the browser to resize the image, and browsers do a really lousy job of it. And with that much difference in size (4288 to 960) the image is going to look like crap.
The maximum usable browser window area with a 1024 width resolution is about 980px with the browser maximized. If there is a sidebar, then you will not be able to have images that wide. With your theme, you would have about 650 usable. If you reduced the padding on the left hand side, you could gain maybe 20px more without things looking really bad.
If you go with a theme with top navigation and ditch the sidebar, you are looking at 960px wide images without horizontal scrolling.