The first step in troubleshooting large (read: wide) images not being rendered is to (using the development version, at this point) try setting landscape orientation. You might also try a different theme (as a test), such as Twenty Twelve, which seems to automatically constrain larger images, even when passing them to the plugin.
There are still some image types which we don't render. The development version has a mechanism for this, but currently only for SVG images (we swap in placeholder text). PNG and JPG should render properly, assuming we can access the image files and that the theme automatically constrains them within the content margins.
Please try the above suggestions first, then if still not satisfied with the result, consider adjusting the image scaling ratio (which defaults to 1.75) downward in small increments (or for testing, in one large increment) to see if overall image scaling might be a better solution for your site's theme.
If still no improvement (or if not enough), then please open a support request for a specific example page, and be sure to list the image sizes, file types, and theme being used.