Can the main content box be adjusted based on image width?

You’d have to set your content widths to something relative — a percentage instead of a number.

I tried working up a variable width theme and decided there were far too many problems because of images and browser differences. I’d suggest that you either use thumbnails or small images with links to the larger version, or else set your widths to accommodate the widest images you plan to use.

For your particular example, I think that image would still be effective resized small enough to fit in your existing content column.

EDIT — there’s another option — look up “overflow” in a CSS tutorial. You could set your image overflow to show even though the content box wouldn’t expand.