Remove post thumbnail in post pages

Hi,

I’m hoping you can help me. I need to remove the post thumbnail/feature image that appears as a masthead image in the posts pages in the fortuna theme. Is there a way and how would I go about this?

If i needed to keep it on some pages, is there also a way to stop the image from scaling up to size, as it makes pictures looks pixelated. Please see this page for reference http://virtualnotdistant.com/silence-in-blank-verse/

I’m not a developer so simple terms would be very helpful!

Thank you very much!

Caroline