Moar img {max-width: 100%;}

See also.

Thanks again, Ethan Marcotte and Richard Rutter.

The images below are all of one that is 1060px x 600px, at full scale.

Here is Blogger's default centered, large layout:

(Used Blogger to host the image because Photobucket resizes anything wider than 1024 px.)

Here it is with style="max-width:100%; border:none; margin:0;"

Here it is with style="max-width:100%; border:none; margin:0; padding:0;", since Blogger (in this template, at least), specifies padding:4px for all images in posts.