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