How to fill “gray thumbnail box” with thumbnail image?

Hi,

I’m trying to figure out how to fill the gray thumbnail box on my homepage with an actual image. If you go to https://www.the-heap.com you will see what I’m talking about. The gray box is right underneath “Hello World”. I know that’s where a thumbnail is suppose to go but when I add a thumbnail, instead of it going in that box it goes beside it. How do I get the thumbnail to go inside that box? I have searched high and low to figure this out and gotten nothing but a headache. Can anyone help?

Thank you.