[help] CSS image size

Hello css gurus,
Im having a little problem with my new template. Im making a custom template for my blog. I'm use class for img, with which I fixed the image size to 600px. If I upload image bigger than 600px. it automatically resize it to 600px. But if I upload image smaller than 600px; eg- 500px; it don't stretch my image to 600px. please let me know how do I do?

hope you understood the prob

 

maybe min-width: 600px; would help..

 

I used following code, Its working, Thanks -

for those who may need it in future