I am posting a tip to help theme developers.
theme users sometimes happen to inserts photos their posts which are too large (meaning - the width of the photo is larger the then width of the post container in the theme).
(assuming the theme-post-container size is 450px)
add the following line of code in the theme style.css:
note of caution: this solution won't work for visitors using IE6, but it's much better then nothing...