How to generate thumbnails when needed only?

Take a look at Otto’s Dynamic Image Resizer plugin

This plugin 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).