Possible to upload 1 image and use it in 2 different sizes?

I am using a custom field to add a thumbnail to each of my posts that appears on my index.php

When a user clicks on one of these I want it to show a larger version of the image on the post page.

I know how to do all this if I were to upload 2 images in the 2 different sizes, but I wondered if there was a way to only upload the larger of the two, and have WordPress/GD create the thumbnail version for me?

Thanks