Alternative to featured image in slider?

Hi,

I noticed that when you a set featured image on a post, it is shown as a thumbnail next to the post and also in the slider, but it is always stretched in my slider. This is because the thumbnail next to the post and the slider image are completely 2 different sizes. What is the best alternative to having a separate size image for the same post to display in the slider? You can see what I’m talking about here: http://www.destinyinsider.com

Great theme guys!