Post features image – that does NOT show in the actual post

Hi guys,

I have a music/lifestyle blog https://www.tinmanlondon.com and I would like to make some small modifications to how images are displayed with each post. As you can see from our homepage all posts have a 150×150 image thumbnail, however we would like the image to be significantly larger when you open the post or on some occasions have no image showing within the actual post. Can anyone advise how we could this please? Is there a Plugin available or does it require physical code change?

Thanks
Lewis