how to prevent scaling up images bigger than original ones in blog page?

Parabola chooses an image from each post and shows it alongside post summary in blog post list which is cool but my problem is the size of this image which has far larger dimensions than the original uploaded one. For example I upload an 200px*200px image but parabola shows that in 800px*800px dimensions which is bad looking. Would you please guide me how I can fix this? thanks in advance and sorry for my poor English!