Gallery setup and header image issues

Two questions I need help with please!

1. Why does my header image not scale properly? I made it’s resolution exactly 1349 x 596 but there is a good slice on the bottom and right being cut off.

2. How do I setup/add a gallery? Specifically, I want a grid/gallery of thumbnails that are actually pages like this wordpress site here: https://www.franklinfotographics.com/gallery/

My site is https://officialoffbeat.com/

Thank you for the help!