How to generate thumbnails when needed only?

I have 1000 images.
How can i make wordpress to generate thumb only when needed. For example home slider will only use 10 images i dont want the other 1000 images have that thumbnail generated as its a waste of space and resources.

There is a way to fire add_image_size only when needed ?

Thanks

UPDATE As you mention is not really add_image_size what it needs to be fired. What it would be great is to fire the image resize when i use the_post_thumbnail(‘slider-thumb’);
Maybe this slowdown the first view of the image but that view is usually generated by me when i actually review the post so i dont care.

So between my posts, slider, blog thumbnauls, portfolio thumbnails etc i got 1000 images and i want just 10 images to be resized for the slider i see a lot of wasted resources to generate the thumbnail size for the other 990 images.

Hope its clear now, sorry for my english

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).