resize images depending on size

Hello,

I am looking for a way to resize images depending on size.
In a list of thumbnails very wide images should be half as high as ordinary images but twice as wide.
I don’t want to register two image sizes, because it is a special case and too many unused images would be generated.
Rather, I would like to switch the width of a registered image size depending on the original image.
Any other solution is also appreciated.

Thank you,
Ramon