Resizing all previous WordPress images

I 开发者_如何学Crecently changed the main column width of my WordPress blog from 500 pixels wide to 600 pixels wide. I've changed medium-sized images in the WP Media Settings to be 600 pixels wide accordingly, but I don't know how to resize all of the previous images I've posted (which are now 100 pixels too small). Does anyone know the best way to do this?

as simple as installing this plugin: http://wordpress.org/extend/plugins/regenerate-thumbnails/