Image Question

Hello,
I’ve been messing around with WordPress and have a quick question. I have and image in a post. I was wondering if anyone knew how I could use the same image but change the dimensions of it on another page. I basically want a small thumbnail picture in my index.php and then the same picture, only larger one the single.php page.( the one with allowing you to write comments)

If anyone can point me in the right direction, it would be greatly appreciated.
Thank you
Simon