'featured image' does not show the correct picture

I am using WordPress 3.5.1, I am trying to use ‘Featured image’ to put the thumbnail next to the post, but i can see the image shows in the frontend is not what I want: it shows this one:
<img width="600" height="240" alt="m&ms" class="alignnone wp-post-image"src="https://mysite.com/blog/wp-content/uploads/mms-600x240.jpg">
But I want this one:
https://mysite.com/blog/wp-content/uploads/mms.jpg
so what is going on here? how could I make it show the proper image:mms.jpg