Featured image trouble

I’m having real trouble sorting out the featured image on my blog at Tom Foster Photography Blog.

It is fine when the screen resolution is a certain size but when the screen size gets below this point, the featured image all of a sudden gets really big to the point of stretching and distorting the original image. Is there any CSS I can use to fix this and maybe keep it aligned left until the screen is much smaller?
I’ve been having trouble with this for ages and would really appreciate help from someone that knows what they’re doing! Thank you!