Hover state for images in 7.1

Site URL: https://cello-guava-wta6.squarespace.com/

I know I must be missing something really simple, but I can't quite figure it out. 

I want to create hover states for the images on the homepage: https://www.screencast.com/t/z9pO7mNIb

I cannot for the life of me figure this out. These are the only options I get with the gallery I inserted: https://www.screencast.com/t/P5bm9gtPVn

So I tried adding a summary wall like this: https://www.screencast.com/t/JnMfttgpTBK
.... and I got this: https://www.screencast.com/t/RpmVYlLY

I don't know how to edit those images, so I tried creating a Gallery page with the book covers I want here: https://www.screencast.com/t/jJhgJGYyuLY
.... but I still can't pull them in. 

Can anyone point me in the right direction? Thank you so much! 

Can you describe in detail hover effect?

Sure thing. I basically just want the same functionality that you seen on this site: https://presentperfectlit.com/
... Here's a video. Thank you for your help! 

Bumping this up to hope to get a response. Thank you! 

You want: change image on hover?

This is for 7.1 It will take two images to do what you want.