Making home page pic a different size than same pic in the gallery

I am trying to figure out how to set the picture on the home page to a different size than the pictures in the actual post. For example, I am starting to do High Resolution posts and the pictures in the gallery are set to 920px width. However, the pictures on the home page are also at 920px. I would like to know how to be able to set the picture on the home page to 500px and when you click on the link to open the gallery, the pictures open up at 920px.

For a better understanding, go to https://pacontreras.com and you can see what I am talking about. Thank You in advance.