Solution:
The following plugin does exactly that:
Dynamic Image Resizer
Changes the way WordPress creates images to make it generate the images only when they are actually used somewhere, on the fly. Images created thusly will be saved in the normal upload directories, for later fast sending by the webserver. The result is that space is saved (since images are only created when needed), and uploading images is much faster (since it’s not generating the images on upload anymore).
The author is WordPress core developer and knows WP code inside out.