Resize thumbnail to fit in WordPress theme?

Hi Fluther!

I’m making a custom theme and I need Wordpress to resize my images on the fly. I’m currently using

the_post_thumbnail(array(208,158));

inside a loop to display the thumbnail. No problem with the actual code, but let’s say the image is 208px in width and 130px in height. WordPress finds no need to resize it to fit the 208×158 frame.

Any WP experts in here? How would I go about doing this? I’m using WordPress 3.1.4.

Thanks! :)
—Davey