[Theme: Oxygen] Resize archive-thumbnail by default

Hello,

I would like to add a featured image of any size to an post and than get an archive-thumbnail of 470/140 by default. If the image is too big, I would like wordpress to shrink the image till it fits an fill up the rest of the 470/140 with white/transparent.
The same with single-thumbnail-featured.

Is this possible?

I already tried changing function.php -> add_image_size( ‘archive-thumbnail’, 470, 140 ) but i wouldn’t work. The images came up to big. I also tried thumbnail editor plugin but wouldn’t.

Any ideas?

I use the latest Version of wordpress (3.9.1) as well as of Oxygen.

My blog is:

Thanks for helping!