Attach featured images in posts,

Hi.
I want a little help about how i can make a function to resize a image in a different resolution used by theme.
IE: the theme’s thumbnails is 210px x 210px, i want a different resolution resize like 620px X 210px.
there is an example: http://burned.p.ht/crossgun-and-bow-bug-what-you-think-about-it/
I want to resize the image to fit the content field.

Any ideas?

ps.: sorry for the bad English, its not my first language.