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 output is the full picture (if I try hard crop with “true” it works so). 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: here
Thanks for helping!