When previewing a post, large images are shown full size outside the page width.
Using this image for example: http://files.myopera.com/white_demon/albums/886891/ice-age-scrat-wallpaper-hi-res.jpg
Gives this on preview:
Using this image for example: http://files.myopera.com/white_demon/albums/886891/ice-age-scrat-wallpaper-hi-res.jpg
Gives this on preview: